Aliphatic Hydrocarbones : Alkanes

Transcription

1 Dr. Salam Ghafour Taher Aliphatic Hydrocarbones : Alkanes Alkanes are fully saturated hydrocarbons, have only C s and H s. Contains single bonds only. Alkanes have the general formula C n H 2n+2 example: CH 4 They can be straight-chained or branched, example: CH 3 CH 2 CH 2 CH 3 vs. (CH 3 ) 2 CHCH 3 Alkanes, and all other organic compounds, are named according the IUPAC system (International Union of Pure and Applied Chemistry)

2 Hybridized Carbon Orbitals of alkanes The four sp 3 orbitals are arranged in a tetrahedral shape. Each hybrid sp 3 orbital contains one electron and is available to bond with another atom. Four hydrogen atoms, each having an s orbital, overlap with each of the sp 3 orbitals. Example CH4 methane Each of these bonds is called a σ (sigma) bond.

3 There are two different ways to arrange four carbons, giving two compounds with molecular formula C 4 H 10, named butane and isobutane. Butane and isobutane are isomers two different compounds with the same molecular formula. Specifically, they are constitutional or structural isomers. Constitutional isomers differ in the way the atoms are connected to each other. 3

4 Carbon atoms in alkanes and other organic compounds are classified by the number of other carbons directly bonded to them. 4

5 Hydrogen atoms are classified as primary (1 ), secondary (2 ), or tertiary (3 ) depending on the type of carbon atom to which they are bonded. 5

6 Nomenclature The name of every organic molecule has 3 parts: 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. 6

7 Common names CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 CH 3 CHCH 2 CH 2 CH 3 n-hexane isohexane CH 3 CH 3 CH 3 CH 2 CHCH 2 CH 3 CH 3 CCH 2 CH 3????? CH 3 neohexane CH 3 CH 3 CHCHCH 3 CH 3????

8 IUPAC rules for naming alkanes: parent chain = longest continuous carbon chain alkane. CH 4 = methane, CH 3 -CH 3 = ethane. branches on the parent chain are named as alkyl groups. CH 3 -CH 2 - = ethyl CH 3 -CH 2 -CH 2 = propyle number the parent chain starting from the end that gives you the lower number for the first branch (principle of lower number). assign locants to the alkyl branches. if an alkyl group appears more than once use prefixes: di, tri, tetra, penta ; each alkyl group must have a locant! the name is written as one word with the parent name last. The names and locants for the alkyl branches are put in alphabetic order (ignore all prefixes except iso) separating numbers from numbers with commas and letters from numbers with hyphens.

9

10

11 Naming Branched Alkanes 1. For branched-chain alkanes, longest chain of carbon atoms is the parent chain and its name is the root name. 2. Name and number each substituent on the parent chain; use a hyphen to connect the number to the name CH 3 CH 3 CHCH 3 2-Methylprop ane If there is one substituent, number the parent chain from the end that gives the substituent the lower number CH 3 CH 3 CH 2 CH 2 CHCH Methylpen tane (n ot 4-methylpentan e)

12 1. Find the parent carbon chain and add the suffix. Note that it does not matter if the chain is straight or it bends. 12

13 Also note that if there are two chains of equal length, pick the chain with more substituents. In the following example, two different chains in the same alkane have seven C atoms. We circle the longest continuous chain as shown in the diagram on the left, since this results in the greater number of substituents. 13

14 If the first substituent is the same distance from both ends, number the chain to give the second substituent the lower number. 14

15 When numbering a carbon chain results in the same numbers from either end of the chain, assign the lower number alphabetically to the first substituent. 15

16 3. Name and number the substituents. Name the substituents as alkyl groups. Every carbon belongs to either the longest chain or a substituent, not both. Each substituent needs its own number. If two or more identical substituents are bonded to the longest chain, use prefixes to indicate how many: di- for two groups, tri- for three groups, tetra- for four groups, and so forth. 16

17 hexanes C 6 H 14 IUPAC names CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 CH 3 CHCH 2 CH 2 CH 3 (n-hexane) (isohexane) n-hexane 2-methylpentane CH 3 CH 3 CH 3 CH 2 CHCH 2 CH 3 CH 3 CCH 2 CH 3 (no common name) CH 3 3-methylpentane CH 3 CH 3 CHCHCH 3 CH 3 (no common name) 2,3-dimethylbutane (neohexane) 2,2-dimethylbutane

18 Alkanes, physical properties non-polar or only weakly polar, cannot hydrogen bond relatively weak intermolecular forces, They have low melting and boiling points due to their weak intermolecular forces (dispersion) They are also less dense than water Alkanes are mostly obtained from crude oil -the crude oil is fractionated based on boiling pt. at room temperature: C 1 C 4 are gases C 5 C 17 are liquids > C 17 are solids

19 Longer or Straight Chains - more interactions along chain. H H H H + - H C H H H H C C + C C H C H - C C H H - H H + H C C H H H H H H H H H H H + H Branching (ball-like) less surface to interact: bpt, mpt, density less H C C C H H H H H HH C C H C - C H C C + - H H H H C + H H H C H C H H H H H H

20 Sources of Alkanes Natural gas 90-95% methane Petroleum Coal gases (bp below 20 C) naphthas, including gasoline kerosene fuel oil lubricating oils (bp above 350 C) asphalt (residue after distillation)

21 Fractions obtained from crude oil The smallest (lightest; at top) hydrocarbon molecules are used as gases. Intermediate hydrocarbon molecules are used in liquid form. Largest (heaviest; at bottom) hydrocarbon molecules as used as solids (e.g. tar). Fractions that condense in each tray are extracted and used for different purposes.

22 Solvents, used in paints, lacquers, and printing inks, and cleaners The Versatility of Petroleum But petroleum-derived organic molecules are also used in an incredible number of other products that include: Petroleum jelly (Vaseline), used in medical products and toiletries Lubricating oils and greases for machinery Petroleum (or paraffin) wax used in candy making, candles, packaging, matches, and polishes Asphalt, used to pave roads and airfields and to make roofing materials and floor coverings Plastics and synthetic rubber, used in packaging, casings, fabrics, bubble gum, etc.

23 Preparation of alkanes a-reduction of an alkyl halide a) hydrolysis of a Grignard reagent (two steps) i) R X + Mg RMgX (Grignard reagent) ii) RMgX + H 2 O RH + Mg(OH)X CH 3 CH 2 CH 2 -Br + Mg n-propyl bromide CH 3 CH 2 CH 2 -MgBr n-propyl magnesium bromide CH 3 CH 2 CH 2 -MgBr + H 2 O CH 3 CH 2 CH 3 propane + Mg(OH)Br

24 b) with an active metal and an acid R X + metal/acid RH active metals = Sn, Zn, Fe, etc. acid = HCl, etc. (H + ) CH 3 CH 2 CHCH 3 + Sn/HCl CH 3 CH 2 CH 2 CH 3 + SnCl 2 Cl sec-butyl chloride n-butane CH 3 CH 3 CH 3 CCH 3 + Zn/H + CH 3 CHCH 3 + ZnBr 2 Br tert-butyl bromide isobutane

25 c. Corey-House synthesis R-X + Li R-Li + CuI R 2 CuLi R 2 CuLi + R -X R R (alkane) (R -X should be 1 o or methyl) This synthesis is important because it affords a synthesis of a larger alkane from two smaller alkyl halides.

26 Alkane, reactions: Alkanes are the only family of organic molecules that have no functional group. Consequently, they undergo very few reactions. The major reactions are: 1. Halogenation 2. Combustion (oxidation) 3. Pyrolysis (cracking)

27 Alkane, reactions: 1. Halogenation R-H + X 2, heat or hv R-X + HX a) heat or light required for reaction. b) X 2 : Cl 2 > Br 2 I 2 c) yields mixtures d) H: 3 o > 2 o > 1 o > CH 4 e) bromine is more selective

28 CH 3 CH 3 + Cl 2, hv CH 3 CH 2 -Cl + HCl ethane ethyl chloride CH 3 CH 2 CH 3 + Cl 2, hv CH 3 CH 2 CH 2 -Cl + CH 3 CHCH 3 propane n-propyl chloride Cl 45% isopropyl chloride 55% gives a mixture of both the possible alkyl halides!

29 In the reaction of alkanes with halogens mechanism: initiating step: 1) X X 2 X propagating steps: 2) X + R H H X + R 3) R + X X R X + X 2), 3), 2), 3) terminating steps: 4) 2 X X X 5) R + X R X 6) 2 R R R

30 2. Combustion C n H 2n+2 + (xs) O 2, flame n CO 2 + (n+1) H 2 O + heat gasoline, diesel, heating oil Alkanes undergo combustion that is, they burn in the presence of oxygen to form carbon dioxide and water. This is an example of oxidation. Every C H and C C bond in the starting material is converted to a C O bondin the product. 30

31 3. Pyrolyis (cracking) alkane, o C smaller alkanes + alkenes + H 2 Used to increase the yield of gasoline from petroleum. Higher boiling fractions are cracked into lower boiling fractions that are added to the raw gasoline. The alkenes can be separated and used in to make plastics.

32 Cycloalkanes Carbons can also bond together to form rings Rings with only C s, H s and single bonds are called cycloalkanes Cycloalkanes have the general formula C n H 2n The smallest is cyclopropane (C 3 H 6 ) - cyclopropane is an unstable molecule - it s forced to have bond angles of 120 between the C s, while they would normally be 109 (the carbons each have 4 electrons groups and should be tetrahedral) The only cycloalkanes with little or no strain are cyclopentane (C 5 H 10 ) and cyclohexane (C 6 H 12 ) Most cycloalkanes are not flat because they prefer tetrahedral geometry

33

34 Naming Cycloalkanes Cycloalkanes are named by adding cyclo- to the parent alkane name Substituents are numbered when there is more than one of them They are numbered starting at the one that gives the others the lowest numbers Or, they are numbered alphabetically when there are only two, or both directions are equal Cl = 2-chloro-1,4-dimethylcyclohexane

35 36

36 Cis and Trans Isomers Because cycloalkanes do not have free rotation around the carbons, it matters on which side of the ring a substituent is relative to other substituents Two substituents on the same side (top or bottom) of the ring are called Cis Two substituents on opposite sides of the ring are called Trans Cis and Trans isomers are stereoisomers; they have the same molecular formula, and the atoms are connected in the same order but are arranged in a different spacial orientation