Revision Guide Unit 2. Module 1 Organic Chemistry

Transcription

1 Revision Guide Unit 2 Module 1 Organic Chemistry

2 Types of formulae

3 Types of formula you need to know 1. Empirical 2. Molecular 3. Displayed 4. Structural 5. Skeletal 6. General

4 DefiniJons empirical formula - the simplest whole number rajo of atoms of each element present in a compound edg CH 2 molecular formula - the actual number of atoms of each element in a molecule, general formula - the simplest algebraic formula of a member of a homologous series, ie for an alkane: C n H 2n+2, structural formula as the minimal detail that shows the arrangement of atoms in a molecule displayed formula as the relajve posijoning of atoms and the bonds between them, all bonds shown skeletal formula as the simplified organic formula, shown by removing hydrogen atoms from alkyl chains,

5 Molecular and empirical formulae There are many ways of represenjng organic compounds by using different formulae. The molecular formula of a compound shows the number of each type of atom present in one molecule of the compound. Molecular formula C 2 H 6 C 6 H 12 O 6 Empirical formula CH 3 CH 2 O The empirical formula of a compound shows the simplest rajo of the atoms present. C 2 H 4 O 2 CH 2 O Neither the molecular nor empirical formula gives informajon about the structure of a molecule.

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8 Displayed formula of organic compounds The displayed formula of a compound shows the arrangement of atoms in a molecule, as well as all the bonds. Single bonds are represented by a single line, double bonds with two lines and triple bonds by three lines. The displayed formula can show the different structures of compounds with the same molecular formulae. ethanol (C 2 H 6 O) methoxymethane (C 2 H 6 O)

9 Structural formula of organic compounds The structural formula of a compound shows how the atoms are arranged in a molecule and, in parjcular, shows which funcjonal groups are present. Unlike displayed formulae, structural formulae do not show single bonds, although double/triple bonds may be shown. CH 3 CHClCH 3 2- chloropropane H 2 C=CH 2 ethene CH 3 C N ethanenitrile

10 Skeletal formula of organic The skeletal formula of a compound shows the bonds between carbon atoms, but not the atoms themselves. Hydrogen atoms are also omiyed, but other atoms are shown. compounds

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13 DefiniJons homologous series is a series of organic compounds having the same funcjonal group but with each successive member differing by CH 2, func>onal group is a group of atoms responsible for the characterisjc reacjons of a compound

14 You need to know How to use the general formula of a homologous series to predict the formula of any member of the series; How to create the general formula of a homologous series Be able to state the names of the first ten members of the alkanes homologous series;

15 Exam quesjon Q1. Crude oil is a source of hydrocarbons which can be used as fuels or for processing into petrochemicals. Octane, C 8 H 18, is one of the alkanes present in petrol. Carbon dioxide is formed during the complete combusjon of octane. C 8 H ½O 2 8CO 2 + 9H 2 O What is the general formula for an alkane?... [Total 1 mark] Q2. Predict the molecular formula of an alkane with 13 carbon atoms.... [Total 1 mark]

16 Model answers 1. C n H 2n+2 [1] ALLOW C n H 2(n+1) IGNORE size of subscripts 2. C 13 H 28 [1]

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23 FuncJonal groups and homologous series A func>onal group is an atom or group of atoms responsible for the typical chemical reacjons of a molecule. A homologous series is a group of molecules with the same funcjonal group but a different number of CH 2 groups. methanoic acid (HCOOH) ethanoic acid (CH 3 COOH) propanoic acid (CH 3 CH 2 COOH) FuncJonal groups determine the payern of reacjvity of a homologous series, whereas the carbon chain length determines physical properjes such as meljng/boiling points.

24 Naming compounds

25 COMMON FUNCTIONAL GROUPS ALKANE ALKENE ALKYNE HALOALKANE AMINE CARBOXYLIC ACID ESTER ACYL CHLORIDE NITRILE ALCOHOL ETHER AMIDE NITRO ALDEHYDE KETONE SULPHONIC ACID

26 I.U.P.A.C. NOMENCLATURE A systemajc name has STEM This is the number of carbon atoms in longest chain bearing the func>onal group PREFIX - This shows the posijon and idenjty of any side- chain subsjtuents SUFFIX - This shows the funcjonal group is present Number of C atoms stem name 1 meth- 2 eth- 3 prop- 4 but- 5 pent- 6 hex- 7 hept- 8 oct- 9 non- 10 dec-

27 Common prefixes 1- methyl 2- methyl 1- ethyl 2- ethyl 1- propyl 2- propyl 1- chloro 2- chloro 1- fluoro 2- fluoro chloro chlorofluoro dichloro trichloro 1- amino 2- amino

28 Common suffixes - ene alkene (double bond) - yne alkyne (triple bond) - oic acid carboxylic acid - ol alcohol - al aldehyde - one ketone - oyl chloride acyl chloride - nitrile nitrile - amide amide

29 Puqng it all together Start with the stem propan Add the funcjonal group and its posijon 1- ol Add any subsjtuent(s) and their posijon(s) 2- amino 2- amino propan- 1- ol

30 Puqng it all together Start with the stem Add the funcjonal group Add any subsjtuent(s) and their posijon(s)

31 Puqng it all together Start with the stem propan Add the funcjonal group oic acid Add any subsjtuent(s) and their posijon(s) 2- methyl 2- methyl propanoic acid

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34 Branching Look at the structures and work out how many carbon atoms are in the longest chain. CH 3 CH 2 CH 3 CH CH 2 CH 3 CH 3 CH 3 CH 2 CH 2 CH 2 CH CH 3 CH 3 CH 3 CH 2 CH 3 CH 2 CH CH CH 3

35 Answers CH 3 CH 2 CH 3 CH CH 2 CH 3 LONGEST CHAIN = 5 CH 3 CH 3 CH 2 CH 2 CH 2 CH CH 3 LONGEST CHAIN = 6 CH 3 CH 3 CH 2 CH 3 CH CH 2 CH CH 3 LONGEST CHAIN = 6

36 NOMENCLATURE - rules Rules - Summary 1. Number the principal chain from one end to give the lowest numbers. 2. Side- chain names appear in alphabejcal order butyl, ethyl, methyl, propyl 3. Each side- chain is given its own number. 4. If idenjcal side- chains appear more than once, prefix with di, tri, tetra, penta, hexa 5. Numbers are separated from names by a HYPHEN e.g. 2- methylheptane 6. Numbers are separated from numbers by a COMMA e.g. 2,3- dimethylbutane

37 Test your understanding CH 3 Apply the rules and name these alkanes CH 2 CH 3 CH CH 2 CH 3 CH 3 CH 3 CH 2 CH 2 CH 2 CH CH 3 CH 3 CH 3 CH 2 CH 3 CH 2 CH CH CH 3

38 Answers Apply the rules and name these alkanes CH 3 CH 2 CH 3 CH CH 2 CH 3 Longest chain = 5 - so it is a pentane stem. CH 3, methyl, group is ayached to the third carbon from one end methylpentane CH 3 CH 3 CH 2 CH 2 CH 2 CH CH 3 Longest chain = 6 - so it is a hexane stem. CH 3, methyl, group is ayached to the second carbon from one end methylhexane CH 3 CH 2 CH 3 CH CH 3 CH 2 CH CH 3 Longest chain = 6 - so it is a hexane stem, CH 3, methyl, groups are ayached to the third and fourth carbon atoms (whichever end you count from), so we use the prefix di 3,4- dimethylhexane

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41 Naming Alkenes Suffix - ENE Length In alkenes the principal chain is not always the longest chain It must contain the double bond Posi>on Count from one end as with alkanes. Indicated by the lower numbered carbon atom on one end of the C=C bond CH 3 CH 2 CH=CHCH 3 is pent- 2- ene (NOT pent- 3- ene) Side- chain Named similar to alkanes. The posijon is based on the number allocated to the double bond CH 2 = CH(CH 3 )CH 2 CH 3 CH 2 = CHCH(CH 3 )CH 3 2- methylbut- 1- ene 3- methylbut- 1- ene

42 Exam quesjon Q1. Draw the skeletal formula for 2- methylpentan- 3- ol. [Total 1 mark]

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44 Isomerism

45 DefiniJons structural isomers are compounds with the same molecular formula but different structural formulae, stereoisomers are compounds with the same structural formula but with a different arrangement in space, E/Z isomerism is an example of stereoisomerism, arising from restricted rotajon about a double bond. Two different groups must be ayached to each carbon atom of the C=C group, cis- trans isomerism are a special case of E/Z isomerism in which two of the subsjtuent groups are the same;

46 What do I need to be able to do? Determine the possible structural formulae and/or stereoisomers of an organic molecule, given its molecular formula.

47 TYPES OF ISOMERISM CHAIN ISOMERISM STRUCTURAL ISOMERISM Same molecular formula but different structural formulae POSITION ISOMERISM FUNCTIONAL GROUP ISOMERISM E/Z ISOMERISM STEREOISOMERISM Same molecular formula but atoms occupy different positions in space. Occurs due to the restricted rotation of C=C double bonds... two forms E and Z (CIS and TRANS) OPTICAL ISOMERISM Occurs when molecules have a chiral centre. Get two nonsuperimposable mirror images.

48 Structural isomerism - chain These are caused by different arrangements of the carbon skeleton. They have similar chemical proper>es These have slightly different physical proper>es Make the structural isomers of C 4 H 10. BUTANE 2-METHYLPROPANE C straight chain C branched

49 Structural isomerism - posijonal Each molecule has the same carbon skeleton. Each molecule has the same func>onal group... BUT the funcjonal group is in a different posijon They have similar chemical proper>es They have different physical proper>es PENT-1-ENE double bond between carbons 1 and 2 PENT-2-ENE double bond between carbons 2 and 3

50 Structural isomerism - FuncJonal group Molecules have same molecular formula Molecules have different func>onal groups Molecules have different chemical proper>es Molecules have different physical proper>es ALCOHOLS and ETHERS ALDEHYDES and KETONES ACIDS and ESTERS

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55 Stereoisomerism Molecules have the same molecular formula but the atoms are joined to each other in a different spacial arrangement - they occupy a different position in 3- dimensional space. There are two types... E/Z isomerism Optical isomerism

56 E/Z isomerism These are found in some, but not all, alkenes These isomers occurs due to the lack of rota>on of the carbon- carbon double bond (C=C bonds) Z Groups/atoms are on the SAME SIDE of the double bond E Groups/atoms are on OPPOSITE SIDES across the double bond CIS and TRANS are a special case of E/Z where the groups on each side of the double bond are the same

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63 OpJcal isomerism These occur when compounds have non- superimposable mirror images The two different forms are known as opjcal isomers or enanjomers. They occur when molecules have a chiral centre. A chiral centre contains an asymmetric carbon atom. An asymmetric carbon has four different atoms (or groups) arranged tetrahedrally around it.

64 Chiral centres There are four different colours arranged tetrahedrally about the carbon atom.

65 Percentage yield and atom economy

66 DefiniJons Percentage yield Percentage yield= Actual mass of products/ Theore9cal mass of products x 100 Atom economy Atom economy= molecular mass of the desired product/sum of molecular massess of all products x 100

67 You need to be able to Explain that addijon reacjons have an atom economy of 100%, whereas subsjtujon reacjons are less efficient Describe the benefits of developing chemical processes with a high atom economy in terms of fewer waste materials Explain that a reacjon may have a high percentage yield but a low atom economy

68 Percentage yield calculajons 1. When calcium carbonate is heated fiercely it decomposes to form calcium oxide and carbon dioxide. CaCO 3 (s) CaO(s) + CO 2 (g) 5.00 g of calcium carbonate produced 2.50 g of calcium oxide. What is the percentage yield of this reacjon? 2. Potassium chloride is made by the reacjon between potassium and chlorine. 2K(s) + Cl 2 (g) 2KCl(s) 4.00 g of potassium produced 7.20 g of potassium chloride. What is the percentage yield of this reacjon? 3. When potassium chlorate is heated strongly it decomposes to produce potassium chloride and oxygen. 2KClO 3 (s) 2KCl(s) + 3O 2 (g) HeaJng 3.00 g of potassium chlorate produced 1.60 g of potassium chloride. What is the percentage yield of this reacjon?

69 Test your knowledge - answers 1. When calcium carbonate is heated fiercely it decomposes to form calcium oxide and carbon dioxide. CaCO 3 (s) CaO(s) + CO 2 (g) 5.00 g of calcium carbonate produced 2.50 g of calcium oxide. What is the percentage yield of this reacjon? 89.3% 2. Potassium chloride is made by the reacjon between potassium and chlorine. 2K(s) + Cl 2 (g) 2KCl(s) 4.00 g of potassium produced 7.20 g of potassium chloride. What is the percentage yield of this reacjon? 94.2% 3. When potassium chlorate is heated strongly it decomposes to produce potassium chloride and oxygen. 2KClO 3 (s) 2KCl(s) + 3O 2 (g) HeaJng 3.00 g of potassium chlorate produced 1.60 g of potassium chloride. What is the percentage yield of this reacjon? 87.9%

70 Atom economy In most reacjons you only want to make one of the resuljng products Atom economy is a measure of how much of the products are useful A high atom economy means that there is less waste this means the process is MORE SUSTAINABLE.

71 Atom economy calculajons Calculate the atom economy for the forma>on of nitrobenzene, C 6 H 5 NO 2 Equa>on C 6 H 6 + HNO 3 C 6 H 5 NO 2 + H 2 O M r Atom economy = molecular mass of C 6 H 5 NO 2 x 100 molecular mass of all products = 123 x 100 = 87.2% An ATOM ECONOMY of 100% is not possible with a SUBSTITUTION REACTION like this

72 Atom economy - calculajons Calculate the atom economy for the prepara>on of ammonia from the thermal decomposi>on of ammonium sulphate. Equa>on (NH 4 ) 2 SO 4 H 2 SO 4 + 2NH 3 M r Atom economy = 2 x molecular mass of NH 3 x 100 molecular mass of all products = 2 x 17 = 25.8% 98 + (2 x 17) In industry a low ATOM ECONOMY isn t necessarily that bad if you can use some of the other products. If this reac>on was used industrially, which it isn t, the sulphuric acid would be a very useful by- product.

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