Chem 240. Alkane. Chapter 2. Dr. Seham ALTERARY nd semester

Transcription

1 Chem 240 Chapter 2 Alkane Dr. Seham ALTERARY nd semester

2 Chapter outlines ydrocarbons hierarchy & chemical formulas. Isomersim. Common name & IUPAC naming system. Nomenclature of Alkane. Physical Properties of Alkanes. General Reactions of Alkanes. Nomenclature of Cycloalkane. Geometric isomerism in cycloalkanes General Reactions of Cycloalkane. Bicycloalkane. Conformantional analysis; Ethane, Butane & Cychexane

3 ydrocarbons Aliphatic Acyclic or Cyclic Aromatic Alkanes Alkenes Alkynes C n 2n+2 C n 2n C n 2n-2 3

4 Alkanes: Alkanes are the first class of simple hydrocarbons that consist only of hydrogen & carbon atoms which are bonded only as carbon-carbon single bonds. The chemical formula for the simple alkanes follows the expression: C n 2n+2 They are also known as the paraffin hydrocarbons, from the latin parum affinis, meaning "little affinity," because these compounds are not very chemically reactive.

5 Isomerism Is the phenomenon of exhibiting two or more compounds with same molecular formula but different physical and chemical properties. The isomerism in organic compounds can be classified broadly into: Isomerism A- Stractural Isomerism (constitutional isomerism) 1- Chain Isomerism 2- Positional Isomerism B- Stereo Isomerism 1- Geometrical Isomerism 1.a- Cis-Trans Isomerism 1.b- E-Z Isomerism 3- Functional Isomerism 4-Tautomerism

6 A- Stractural Isomerism (constitutional isomerism): 1- Chain Isomerism The chain isomerism arises due to different arrangements of carbon atoms leading to linear and branched chains. The chain isomers have almost similar chemical properties but different physical properties. Example Two chain isomers possible with the molecular formula, C Linear Branched n-butane Isobutane or 2- methylpropane

7 2- Positional Isomerism The positional isomerism arises due to different positions of side chains, substituents, functional groups, double bonds, triple bonds etc., on the parent chain. Example_1 Propyl chloride and isopropyl chloride are the positional isomers with the molecular formula, C 3 7 Cl. These isomers arise due to difference in the position of the chloro group on the main chain. Propyl Chloride or 1-Chloropropane Isoproyl chloride or 2-Chloropropane Example_2 In the following positional isomers i.e., but-1-ene and but-2-ene the position of double is different but they have same molecular formula, C 4 8 But-1-ene But-2-ene

8 3- Functional Isomerism The functional isomers have the same molecular formula but possess different functional groups. The functional isomers show different physical as well as chemical properties. Example The dimethyl ether is the functional isomer of ethyl alcohol. Both have the same molecular formula, C 2 6 O. owever they have different functional groups. Ethylalcohol Dimethylether

9 4- Tautomerism Tautomerism refers to the dynamic equilibrium between two compounds with same molecular formula. Example Keto-enol tautomerism: The carbonyl compounds containing at least one α-hydrogen atom exhibit keto-enol tautomerism. The carbonyl group can be converted to enol form due to transfer of one of the α-hydrogen onto the oxygen atom.! Acetone Keto form Prop-1-en-2-ol Enol form It is very important to note that the tautomers are not the resonance structures of same compound.

10 Does the molecular formula fully identify a substance? Compound Boiling Point ( C) Melting point ( C) Molecular formula I C 4 10 II C 4 10 Compound I & II are different although they have the same chemical formula. They are called structural or constitutional isomers. Compound (I) Compound (II) C 3 C3 C 3 C 2 C 2 C 3 n- Butane 3 C C Isobutane

11 Q:What are all the possible structure for pentane C 5 12? Alkane beyond butane are capable of structural isomerism. Thus, there are three isomers for pentanes; C C 3 C 2 C 2 C 2 C 3 n- pentane 3 C C 3 C3 C C 3 C 3 C 3 CC 2 C 3 Isopentane neopentane

12 Q: ow many isomers of hexane; C 6 14 are there?

13 The omologous Series of Alkanes C n 2n+2 : If we keep adding C 2 groups, it is possible to built an infinite series of compounds, called a homologous series. omologous series Each individual member differs from its next neighbor by constant value, equal to C 2 in this case. IUPAC names of the first 10 alkanes

14 Classes of Carbons & ydrogens: Primary (1 ) Carbon Secondary (2 ) Carbon Tertiary (3 ) Carbon Quaternary (4 ) Carbon R-C 3 R-C 2 -R R" C R R' R"' R" C R R' Is normally bonded to one other carbon. The carbon is bonded to two other carbons. The carbon is bonded to three other carbons. The carbon is bonded to four other carbons.

15 ydrogens can also be refered to 1, 2 and 3 according to the type of carbon they are bonded to. A primary hydrogen (1 ) is attached to a primary carbon. A secondary hydrogen (2 ) is attached to a secondary carbon. A tertiary hydrogen (3 ) is attached to a tertiary carbon.

16 IUPAC naming system International Union of Pure and Applied Chemistry IUPAC logo The IUPAC system of nomenclature was established at the end of the 19th century in order to have a common method of naming compounds. In general compounds are classified and named by consideration of: a) the number and types of atoms that are present, b) the bond types in the molecule, and c) the geometry of the molecule.

17 Alkyl groups An alkyl group is an alkane which is formed from the removal of a hydrogen. The symbol R is used to represent an alkyl group. Alkyl groups have have the general formula C n 2n+1. They have one less hydrogen atom than a corresponding alkane. Naming branched or side-chain (alkyl group): Remove From Methane C 1 -

18 From Ethane C 2 - From Propane C 3 1 s 3 CC 2 C 3 2 s Remove 1 Remove 2 3 CC 2 C 2 n-propyl group 3 CCC 3 Isopropyl group

19 1 s From Butane C 4 Remove 1 3 CC 2 C 2 C 2 n-butyl group 3 CC 2 C 2 C 3 2 s Remove 2 3 CCC 2 C 3 sec-butyl group 1 s C 3 3 CCC 3 From Isobutane C 4 C Remove CCC 2 Iso-butyl group 3 s Remove 2 C 3 3 CCC 3 tert-butyl group

20 C 3 C 2 C 2 C 2 C 2 n-pentyl From Pentane C 5 3 C 3 C C C 2 C 2 Iso-pentyl C 3 C 2 C 2 C 2 C 3 3 C Sec-pentyl C 3 C C2 C 3 neopentyl 3 C C 2 t-pentyl or C 3 C C 3 tert-pentyl! Note that; when the number of carbon atoms is increased to 5, a new type of side chain appears which is called neo-, The prefix neo means new.

21 The prefix iso means same It can be used to name group content the following features; C 3 C(C 2 ) n C 3 n = Isopropyl When n = 0, 1, 2, 3,.. 3 C C 3 C n = Isobutyl 3 C 3 C C C 2 n = Isopentyl 3 C 3 C C C 2 C 2 n = Isohexyl 3 C 3 C C C 2 C 2 C 2

22 The prefix sec when the chain is attched through sec- C. Starts from n = 1, I.e. from butane C 4 C 3 C(C 2 ) n C 3 R C 3 C(C 2 ) 1 C 3 R n = 1 sec-butyl C 3 C(C 2 ) 2 C 3 R n = 2 sec-pentyl C 3 C(C 2 ) 3 C 3 R n = 3 sec-hexyl

23 Nomencluture Most organic compounds are known by two or more names: the older unsystematic names, which are referred to as common or trivial, and IUPAC names. The name of every organic molecule has 3 parts: Prefix Parent Suffix 1. The parent name indicates the number of carbons in the longest continuous chain. 2. The suffix indicates what functional group is present. 3. The prefix tells us the identity, location, and number of substituents attached to the carbon chain.

24 The IUPAC System of Nomenclature A) For unbranched alkanes They are named according to the number of carbon as they follow the homologous series. Number of Atomes Prefix Name Chemical formula 1 Meth Methane C 4 2 Eth Ethane C Prop Propane C But Butane C Pent Pentane C ex exane C ept eptane C Oct Octane C Non Nonane C Dec Decane C Undec Undecane C Dodec Dodecane C 12 26

25 B) For Branched alkanes 1. Determine the longest continuous chain, and name it as parent or principal chain. Note that; the longest continuous chain is not necessarily straight. It may be in various directions. Ethylhexane Propylpentane

26 2. If two or more chains within structure have the same length, choose the chain with greater number of branches as principal chain.

27 3. Number the carbons in the principal chain (parent chain) starting from whichever end will give the lowest number for the point of attachment of the substituent. Example 3-Ethylhexane 4-Ethylhexane

28 4- If there are two substituents at equal distance from the two ends of the chain, use the alphabet to decide how to number.

29 6. If the same alkyl substituent occurs more than once on the parent carbon chain, the prefix di-, tri-, tetra-, penta-, and so on, are used to indicate two, three, four, five and so on. 7. If there are two substituents on the same carbon, assign the same number.

30 8. If different alkyl substituents are attached on the parent carbon chain, they are attached on the parent carbon chain, in alphabetical order. 4-Ethyl-2,2,7-trimethyloctane 3,3-diethyl-4-methyl-5-propyloctane

31 The Names for Some of The More Common nonalkyl subtituents are: F Fluoro Cl Br Chloro Bromo NO 2 N 2 Nitro Amino CN Cyano I Iodo 9. If substituents other than alkyl groups are also present on the parent carbon chain, then all the substituents are named in alphabetical order. 3-Bromo-2-chloro-4-methyl-pentane

32 ! Note that; In writing the name: a. The substituted groups are listed first by numbers indicating its location on the principal chain and named as alkyl group. b. The principal chain is indicated by the last part of the compound s name suffix = [alkane]. c. The principal chain is numbered from the end that gives the substituents as low a number as possible. d. Numbers are separated from letters by hyphens; (-). e. Numbers are separated from other numbers by commas; (,). f. If the same substituent occurs twice on the same carbon, the number is repeated. g. The numerical prefix di-, tri-, tetra-,..etc as well as the prefix sec-, tert-, are ignored in alphabetizing. h. The prefix iso-, neo-, and cyclo- are considered with alphabetizing substituent groups.

33 Example (1): Write the structural formula of the compound whose IUPAC name is 2,5,5-trimethyl-4-ethylheptane. 1) Parent chain (heptane). 2) Number the carbon starting from either ends 3) There are 3 Me groups; two on C 5 & one on C 2. 4) There are 1 Eth group on C 4. 5) Add the missing s.

34 Example (2): The incorrect name of a compound is 2,2-diethylbutane. Write the structural formula and the correct name of the compound. 1) Parent carbon chain is called butane. 2) Number the chain. 3) There are two ethyl substituents on carbon 2. 4) The longest continuous chain is not C 4 it s C 5 ; i.e. pentane. 5) Add the missing s & rename the compound. 3-Ethyl-3-methylpentane

35 Physical properties of Alkanes Generally they are the properties that can be observed without the compound undergoing chemical reaction. 1- Physical state Alkanes occurs at room temperature as gases, liquid and solids. Alkanes from C 1 -C 4 are gases. Most alkanes from C 5 -C 17 are liquids. Alkanes from C 18 and larger are wax-like solids. 2- Solubility Alkanes are nonpolar compounds. Their solubility characteristic may be predicted by the common rule like dissolves like compounds. Thus, alkanes are soluble in the nonpolar solvents carbontetrachlorid; CCl 4 and benzene: C 6 6, but they are insoluble in polar solvents such as water 2 O.

36 3- Boiling Points (BP) The boiling point for normal hydrocarbons increase with increasing molecular weight. Why??? The molecules become larger there are more forces of attraction between them, and more energy is needed to go from liquids to gaseous state. Amange isomeric alkanes, the straight-chain compound has highest boiling point. The greater the number of branches the lower boiling point, as the surface area decreases causing the intermolecular forces of attraction are diminished & less energy to go from liquids to gaseous state.

37 4- Melting points (MP) The melting points of alkanes also increase with increasing molecular weight.! Note that: there is no regularity in the change in melting points with the number of carbon atoms. Exercise : Arrange the following compounds in order of increasing boiling points: n-hexane, 2,2-dimethylbutane, 2-methylpentane.

38 Alkane Preparation I.Catalytic hydrogenation II. Reduction of Alkyl alides III. Coupling Reactions a.ydrolysis of a Grignard Reagent b.reduction by Metal and Acid a.wurtz Reaction b.corey-ouse Synthesis

39 I- Catalytic hydrogenation In the presence of a metal catalyst such as palladium (Pd), platinum (Pt), nickel (Ni), or rhodium (Rh) and under slight pressure of hydrogen gas hydrogen will add to alkenes to give alkanes. General Equation C n 2n 2 Alkene Examples C 2 C Catalyst ( Pt, Pd, Ni or Rh) Ni C n 2n+2 Alkane C 3 C 3 2 Pd

40 ! Note that; Alkynes undergo catalytic hydrogenation with the same catalysts used in alkene ydrogenation proceeds in a stepwise fashion, forming an alkene first, which undergoes further hydrogenation to an alkane. Example R C C R' 2 Pt R C C R' 2 Pt R C 2 C 2 R' II. Reduction of Alkyl alides a) ydrolysis of a Grignard Reagent Organic compounds in which a metal atom is directly linked to carbon atom are known as organometallic compound. Alkyl or aryl magnesium halide (R-MgX) are also called Grignard reagents or organometallic compounds.

41 Grignard reagent on double decomposition with water having active (the hydrogen attached on O are known as active hydrogen) give alkane. General Equation Dry ether R X + Mg R MgX 2O R Examples C 3 C 2 Br Mg C 3 C 2 MgBr Dry ether 2 O C 3 C 3 + Mg(O)Br Br Mg MgBr 2 O Dry Ether Sec-butyl bromide Sec-butyl magnesium bromide n-butane

42 b) Reduction by Metal and Acid Reduction of an alkyl halide, either via the Grignard reagent or directly with metal and acid, involves the replacement of a halogen X atom by a hydrogen atom. In either case, the carbon skeleton remains intact. The source of hydrogen may obtained from mineral acid or from LiAl 4. General Equation Examples Alkane C 3 C 2 C 2 C 2 Br Zn/ + C 3 C 2 C 2 C 3 C 3 C 2 C 2 C 2 Br 1) LiAl 4 /Ether 2) 3 O + C 3 C 2 C 2 C 3

43 a) Wurtz reaction * In Wurtz reaction, two alkyl halide molecules are coupled in presence of sodium metal in anhydrous ether or (TF) Tetrahydrofuran to form a new carbon carbon bond and thus by giving a symmetrical alkane. * The Wurtz reaction is limited to synthesis of symmetrical alkanes with even number of carbon atoms only. The number of carbons in the alkane is double that of alkyl halide (n ---> 2n type reaction) General Equation III. Coupling Reactions 2R X + 2 Na Dry Ether R R + 2 NaX Examples 2C 3 Br + 2Na Dry Ether 3 C C 3 + Symmetrical Alkane 2NaBr

44 b) Corey-ouse synthesis The synthesis of alkane by the so-called Corey-ouse synthesis, includs the coupling of alkyl halides(r'x) with organometallic compounds (R 2 CuLi); Gilman reagent in dry ether. An alkane of higher number carbon than the starting material is obtained as a result of new C-C bond formation. General Equation ( C 3 ) 2 CuLi R 2 CuLi + R'-X R-R' + RCu + LiX It it a versitle method for the synthesis of all types of alkanes, that is, symmetrical or unsymmetrical, in high yields. Examples Gilman reagent C 3 Br Dry Ether C 3 C 2 Br Dry Ether C 3 C 3 + C 3 Cu + LiBr Ethane, Symmetrical alkane C 3 C 2 C 3 + C 3 Cu + LiBr Propane, Unsymmetrical alkane

45 Reactions of Alkanes Compared with other classes of compounds, saturated hydrocarbons undergo very few reactions. The two important reactions are halogenation and combustion. 1- alogenation alogenation is a typical substitution reaction of alkanes. Only chlorine; Cl 2 and bromine; Br 2 react readily with alkane under high temperature ( ) or under the influence of ultraviolet light. The reaction is illustrated by the general equation: heat or UV light R + X 2 RX + X! Note that: X 2 = Cl 2 or Br 2 Fluorine; F 2 reacts explosively with alkanes and is thus unsuitable reagent. Iodine; I 2 is too unreactive and is not used in the halogenation of alkanes.

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47 Example: The chlorination of alkanes usually gives a mixture of products because more than one hydrogen can be substituted by chlorine atom. Chlorination of methane: heat or UV light C 4 + Cl 2 CCl 4 + Cl excess The reaction proceed in a stepwise manner. (1) (2) C C + Cl Cl Cl + Cl Cl heat or UV light heat or UV light C Cl Chloromethane (Methyl chloride) Cl C Cl + Cl + Cl Dichloromethane (Methylene chloride)

48 (3) Cl C Cl + Cl Cl heat or UV light Cl Cl C Cl + Cl Trichloromethane (Chloroform) (4) Cl Cl C Cl + Cl Cl heat or UV light Cl Cl C Cl + Cl Cl Tetrachloromethane (Carbon tetrachloride) Both methane & ethane give only one monochlorinated product when react with Cl 2 because both own equivalent hydrogens. C 3 C 3 + Cl Cl heat or UV light C 3 C 2 Cl + Cl chloroethane

49 Incase of higher alkanes, a mixture of all possible monochloro alkane is obtained. Example Chlorionateation of propane Two monochloronated products are formed. C 3 C 2 C 3 Propane UV light UV light C 3 C 2 C 2 1-Chloropropane minor 43 % C 3 CC 3 Cl Why?? Cl 2-Chloropropane major 57%

50 The order of hydrogen replacement (reactivity) is: 3 > 2 > 1 Free Radical are more stabile if they are highly substituted

51 Exercise Predict the major product for the following reaction. + Br 2 heat or UV light??? Solution + Br 2 heat or UV light Br 2-bromo-2-methyl propane. Major product. + Br 1-bromo-2-methyl propane. Minor product.

52 2- Combustion The combustion of carbon compounds, especially hydrocarbons, has been the most important source of heat energy for human civilizations throughout recorded history. When ignited in the presence of excess oxygen, alkane are oxidized to carbon dioxide and water. Examples: C 4 + 2O 2 CO O kcal/mol C /2 O 2 CO O kcal/mol C O 2 CO O kcal/mol! Note that: About 160 kcal of heat is liberated for each methylene group added to the hydrocarbon chain.

53 3- Pyrolysis (craking) The conversion of a compound into smaller fragments in the absence of air through the application of heat. It is different from combustion. It happens in the absence of air and hence oxidation of compounds does not take place. Generally, pyrolysis of alkanes is also named as cracking. Note that: Cracking of alkanes follows a free radical mechanism

54 Understanding Questions Q1. Which radical is most stable? A) B) C) D) Q2. Which is the main product upon photo-chlorination of methylbutane with Cl 2? A) B) C) D) Q3. Which of the following alkanes would have the highest boiling point? A) B) C) D)

55 Q4. In chorination ethane, C26, the product we want is C25Cl. Which one of the following experimental conditions is likely to give the best yield of mono-chlorinated product? Explain why. A) C Cl 2 UV light B) C Cl 2 Dark, room temp C) C Cl 2 (excess) D) C 2 6 (excess) + Cl 2 UV light UV light Q5. Which of the following is the mean product when 2 moles of butane reacts with 2moles of sodium in dry ether? A) n-pentane B) n-hexane C) n-heptane D) n-octane Q6. Which of the following is the most stable free radical. A) B) C) D)

56 Cyclo alkanes Cycloalkanes are saturated hydrocarbons that exist in the form of a ring. Cycloalkanes with a ring of three and four membered ring are very reactive. igher cycloalkanes are unreactive, for this reason they are also called cycloparaffins. The empirical formula is: C n 2n or (C 2 ) n

57 1- Nomenclature a-unbranched rings The system for naming this class of compound is straightforward: by adding the prefix cyclo- to the name of the open-chain hydrocarbon. Examples (Cyclopropane) (Cyclobutane) (Cyclopentane) (Cyclohexane) (C 3 6 ) (C 4 8 ) (C 5 10 ) (C 6 12 )

58 b.1-branched rings When only one substituent is attached to the ring, name the substituent first then the name of the ring. (No need to give a number) Examples Methylcyclopropane Ethylcyclobutane Cl Isopropylcyclohexane Chlorocyclopentane

59 If two or more substituents are attached to the ring, their positions are specified by numbers. Examples 1,3-dimethylcyclohextane (Correct) 1,5-dimethylcyclohextane (incorrect) 1-Ethyl-3-methylcyclopentane

60 b.2-branched rings When a single ring system is attached to a single chain with a greater number of carbon atoms. or When more than one ring system is attached to a single chain, then it is appropriate to name the compounds as Examples cycloalkylalkanes. 1-Cyclobutylpentane 1,3-Dicyclohexylpropane

61 2- Geometric isomerism In open-chain alkane very little energy is needed to twist around the carbon-carbon bond, and we say there is free rotation. C C In contrast, in cycloalkanes the carbons are held togather in a ring, too much energy is needed for rotation, and we say there is no free rotation. The lack of free rotation of a single-bonded carbons in a ring produces a kind of isomerism called geometric isomerism.

62 A cycloalkane with two substituents on different carbons of the ring can exist as a cis isomer or trans isomer. In the cis isomer the two substituents are on the same side of the ring. cis-1,2-dimethylcyclopropane. In the trans isomer the two substituents are on opposite side of the ring. trans-1,2-dimethylcyclopropane.

63 Examples trans-1,2-dimethylcyclopentane. cis-1,2-dimethylcyclopentane. trans-1,3-dibromocyclohexane. cis-1,3-dibromocyclohexane.

64 Reaction of cycloalkanes 1-Ring less stable (C-3 & C-4): (I) Addition of /O. Cyclopropane and cyclobutane are exceptions from open-chain alkanes. As they undergo ring-opening and give addition reaction. 2 SO 4 / 2 O C 3 C 2 C 2 O (II) Addition of Br and I. Cyclopropane reacts with concentrated Br or I to yield 1-bromopropane and 1-iodopropane respectively. (cyclobutane and higher member do not give this reaction). I C 3 C 2 C 2 I

65 (III)Addition of ydrogen. Cyclopropane and cyclobutane react with hydrogen in the presence of Ni catalyst to give propane and n-butane respectively. 2 / Ni eat or UV C 3 C 2 C 2 C 3 (IV) Addition of Cl 2 and Br 2 Cyclopropane or cyclobutane react with Cl 2 or Br 2 in the dark to form addition product. CCl 4 is used as solvent. Br 2 / CCl 4 dark BrC 2 C 2 C 2 C 2 Br

66 2-Ring more stable C-5 & C-6 (I) Substitution of Cl 2 and Br 2 Cycloalkanes reacts with chlorine and bromine in the presence of UV light to give substitution products. Cl (1) Cl 2 UV or eat (2) C 3 Br 2 UV or eat C 3 Br

67 Types of Bicycoalkane Bicycoalkane Spirocyclic Compounds Fused bicyclic Compounds Bridged bicyclic Compounds spiro In spirocyclic compounds, the two rings share only one single atom, the spiro atom, which is usually a quaternary carbon. Fused In fused bicyclic compounds, two rings share two adjacent atoms. In other words, the rings share one covalent bond Bridged bicyclic In bridged bicyclic compounds, the two rings share three or more atoms, separating the two bridgehead atoms by a bridge containing at least one atom.

68 Nomenclature of Bicycloalkane Naming compounds containing two fused or bridged rings as bicycloalkanes by using the name of alkane corresponding to the total number of carbon atoms in the rings as parent name. Explaination Bicyclo[2.2.1]heptane

69 Explaination Bridgehead Are the atoms at which two rings are joined in a bicyclic compounds. They may be adjacent or not adjacent atoms A fused bicyclic compound Bridgehead carbons (*) at adjacent positions Bridgehead carbons (*) not adjacent A bridged bicyclic compound

70 Nomenclature of Bicycloalkane To name bicyclic alkanes, follow the following steps: 1- Count the total number of carbons in the entire molecule. 8 carbons = octane 5 carbons = pentane 2- Count the number of carbons between the bridgeheads. Then place in brackets in descending order. Examples: [3,2,1] or [1,1,1] 3- Place the word bicyclo at the beginning of the name.

71 Example: Name the following compound by the IUPAC system for nomenclature: 1- The total number of carbons is 7; so the parent name is heptane 2- The number of carbons between bridgeheads are ; 2,2 and 1. i.e [2,2,1] 3- The name of the compound is : Bicyclo [2,2,1] heptane

72 Examples Nomenclature of Bicycloalkane The total number of carbons is 4 & Functional group is an alkane; i.e Butane The bridgehead carbons are at adjacent positions; i.e. [1,1,0] By adding the prefix Bycyclo- the IUPAC name is: Bicyclo[1.1.0]butane The total number of carbons is 10; & Functional group is an alkane i.e Decane The bridgehead carbons are not adjacent; i.e. [4,3,1] By adding the prefix Bycyclo- the IUPAC name is: Bicyclo[4,3,1]decane

73 In case of Substituent Bycicloalkane. If substituents are present follow the steps below: 1) Name the Parent 2) Number the carbon starting from a bridgehead, longest bridge first & shortest bridgehead last 3) Locate and name the substituent Example: 8-Methyl bicyclo[3.2.1]octane 6-Methyl bicyclo [4.2.1]octane

74 For each of the following structure provide the correct IUPAC name:

75 For each of the following structure provide the correct IUPAC name: Bicyclo[2.2.2]octane Bicyclo[3.3.3]undecane Cl 5-Methyl-bicyclo[2.1.0]pentane 2-Chloro-bicyclo[1.1.1]pentane

76 Conformations and Conformational Analysis of Alkanes Conformations are different arrangements of atoms that are interconverted by rotation about single bonds. Conformations Acyclic Alkanes Cyclic Alkanes Ethane Cyclohexane Butane

77 Conformations and Conformational Analysis of Acyclic Alkane Steriochemical Structure of Ethane

78 1- Conformations and Conformational Analysis of Ethane Two groups bonded by only a single bond can undergo rotation about that bond with respect to each other. The temporary molecular shapes that result from such a rotation are called conformations of the molecule. Each possible structure is called conformer. An analysis of the energy changes that occur as a molecule undergoes rotations about single bonds is called a Conformational analysis.

79 Conformations in Ethane While there are an infinite number of conformations about any sigma bond, in ethane two particular conformers are noteworthy and have special names. Ethane is the simplest hydrocarbon that can have distinct conformations. Two, the staggered conformation and the eclipsed conformation

80 Energy Some important definitions: Torsional Strain An energy increase caused by the eclipsing of bonds. Strain Any increase in energy of a molecule, particularly an increase relative to that which would normally be expected. Eclipsing The relationship between two bonds when the dihedral angle is zero. Conformational Energy Diagram A plot of energy, E (vertically) vs. dihedral angle, theta (horizontally). It should include the structures, energies, and names of the energy minima and maxima. dihedral angle

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82 The certain types of structural formulas are: Staggered conformation Eclipsed conformation dash-wedge Projection Sawhorse Projection Newman Projection The dihedral angle between these hydrogens is 180 C The dihedral angle between hydrogens is 0 C

83 Gauche & Anti Relationships in Ethane Staggered conformations Eclipsed conformation Two bonds are gauche when the angle between them is 60 C Two bonds are anti when the angle between them is 180 C Two bonds are gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.

84 The potential energy diagram of the conformations of ethane shows that the staggered conformation is more stable than eclipsed by 12 kjmol -1.

85 2- Conformations and Conformational Analysis of Butane Butane and higher molecular weight alkanes have several C C bonds, all capable of rotation. The conformational possibilities increase as alkanes become larger. The rotation about the C(2) C(3) bond in butane is shown below dash-wedge Projection Newman Projection

86 Six different conformations of butane It takes six 60 rotations to return to the original conformation Gauche: The relationship between two atoms or groups whose dihedral angle is more than 0 o (i.e., eclipsed) but less than 120 o (i.e., the next eclipsed conformation). A conformation which has one or more gauche interactions is can be called a gauche conformation.

87 A plot of potential energy against rotation about the C(2) C(3) bond in butane is shown below. Staggered conformations 1,3 and 5 are at energy minima. Anti conformations 1 is lower in energy than gauche conformations 3 and 5, which possess steric strain. Eclipsed conformations 2,4 and 6 are at energy maxima. Eclipsed conformations 4, which has additional steric strain due to two eclipsed C 3 group, is highest in energy. Conformation Analysis of Butane Less stable More stable More stable

88 Chair boat chair most stable unstable most stable 3- Conformations and Conformational Analysis of Cyclohexane Why? Cyclohexane has several conformational structures such as: 1-Chair Confromation 2-Boat Confromation

89 Chair conformation most stable Boat conformation most stable

90 Chair Cyclohexane The chair conformation, which is more stable than any possible other conformation. The chair conformation is so stable because it eliminates angle strain (all C C C angles are ), and torsional strain (all hydrogens on adjacent C atoms are staggered). Boat Cyclohexane The boat conformation is destabilized by torsional strain because the hydrogens on the four carbon atoms in the plane are eclipsed.

91 The boat form of cyclohexane is less stable than the chair for two reasons: Flagpole s 1) The proximity of the flagpole s causes steric strain. 2) Eclipsing interaction between s cause torsional strain.

92 ow to Draw Cyclohexane? Chair Conformation

93 Axial and Equatorial Bonds in Cyclohexane The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions. Axial Bonds The six axial bonds, one on each carbon, are parallel and alternate up-dawn. Equatorial Bonds: The six equatorial bonds, one on each carbon, come in three sets of parallel lines. Each set is also parallel to ring bonds. Equatorial bonds alternate between sides around the ring. Completed cyclohexane:

94 Conformational Mobility of Cyclohexane Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip An important conformational change in cyclohexane involves ringflipping. Ring-flipping is a two-step process. As a result of a ring flip, the up carbons become down carbons, and the down carbons become up carbons. Axial and equatorial atoms are also interconverted during a ring-flip. Axial atoms become equatorial atoms, and equatorial atoms become axial atoms.

95 Conformations of Substituted Cyclohexanes 1) Conformations of Monosubstituted Cyclohexanes The two conformations of cyclohexane are different, so they are not equally stable. Larger axial substituents create destabilizing (and thus unfavorable) 1,3-diaxial interactions. In methylcyclohexane, each unfavorable,c 3 interaction destabilizes the conformation by 0.9 kcal/mol, so Conformation 2 is 1.8 kcal/mol less stable than Conformation 1.

96 1,3-Diaxial Interactions Difference between axial and equatorial conformers is due to steric strain caused by 1,3-diaxial interactions ydrogen atoms of the axial methyl group on C 1 are too close to the axial hydrogens three carbons away on C 3 and C 5, resulting in 7.6 kj/mol of steric strain In methylcyclohexane, each unfavorable,c 3 interaction destabilizes the conformation by 0.9 kcal/mol, so Conformation 2 is 1.8 kcal/mol less stable than Conformation 1.

97 2) Conformations of Disubstituted Cyclohexanes In 1,1-dimethylcyclohexane, the two conformers are identical and present in equal concentration. 1,1-dimethylcyclohexane For cis-1,2-disubstitution, one of the substituents must be equatorial and the other axial; in the trans-isomer both may be equatorial. cis-1,2-dimethylcyclohexane Trans-1,2-dimethylcyclohexane

98 Because of the alternating nature of equatorial and axial bonds, the opposite relationship is true for 1,3-disubstitution (cis is all equatorial, trans is equatorial/axial). cis-1,2-dimethylcyclohexane Trans-1,2-dimethylcyclohexane

99 Finally, 1,4-disubstitution reverts to the 1,2-pattern cis-1,4-dimethylcyclohexane Trans-1,4-dimethylcyclohexane

100 Cis-trans isomerism and Conformational Structure of Disubstituted Cyclohexane Relationship between substituents Carbons

101 Understanding Questions Which of the below structures is bicyclo[2.1.1]hexane? A) B) C) D) What is the most stable rotational conformation of butane? A) Anti-conformation B) Gauche C) Eclipsed D) staggered Which cyclohexane conformation has the highest energy? A) Chair B) Boat C) Twist-Chair D) Twist-Boat

102 Thank You For Listening