3.7 Organic naming and Isomerism continued

homologousseries

functional group prefix / suffix(* = usual use) example

aldehydessuffix -al

prefix formyl-

ketonessuffix* -oneprefix oxo-

carboxylic acidssuffix -oic acid

nitrilessuffix -nitrile

prefix cyano-

aminessuffix* -amineprefix amino-

esters-yl –oate

Acyl chloride-oyl chloride

Amide-amide

Acid Anhydrides-oic anhydride

3.7 Organic naming and Isomerism continued

When compounds contain more than one functional group, the order of precedence determines which groups arenamed with prefix or suffix forms. The highest precedence group takes the suffix (and the lowest number on the carbonchain), with all others taking the prefix form. However, double and triple C-C bonds only take suffix form.Order of priority highest first:Carboxylic acids >carboxylic acid derivative>nitriles>aldehydes>ketones>alcohols>amines>alkenes>halogenoalkanes

1

See chapter 3.1 for basic naming of organic molecules. This chapter extends thenaming for functional groups met in next few chapters

C

O

HC C

O

H

H

H

H

C

O

C C

O

C

H

H

H

H

H

H

C

O

OHC C

O

OH

H

H

H

C N C

H

H

C

H

H

C NH

C NH2H

CC

H

NH2

H

C

H

H

H H

C

O

O

C

O

Cl

C

O

NH2

CH3 C

O

NH2

CH 3 C

O

Cl

R C

O

O

CR

O

C C

O

O

H

H

H

C

H

H

H

CH3 C

O

O

CCH3

O

ethanal

Propanone

Ethanoic acid

Propanenitrile

PropylamineOr propan-1-amine

methylethanoate

ethanoylchloride

ethanamide

Ethanoic anhydride

Dr.Ashar Rana www.chemistryonlinetuition.com [email protected]

2

C

H

H

H C

H

H

C

H

H

NH2

AminesThese end in –amine.There is, however, rather confusingly twoways of using this suffix.The exam board tend to use the commonversion where the name stem ends in -ylpropylamine.The IUPAC version of the same chemicalis propan-1-amine. (This is used in thesame way as naming alcohols)

If there is another priorityfunctional group as wellas the amine group thenthe prefix amino is used.propylamine

Or propan-1-amine 2-aminopropanoic acid.

CH3

CH C

O

O H

NH2

If the amine is secondary and has two alkylgroups attached to the nitrogen, then each chainis named and the smaller alkyl group is precededby an –N which plays the same role as a numberin positioning a side alkyl chain

CH3CH2CH2NHCH3N-methylpropylamine (common name)N-methylpropan-1-amine (IUPAC name)

Diethylamine (common name- does not use N if chains aresame length)N-ethylethanamine (IUPAC name does still use N)

CH3CH2

NHCH2

CH3

C

O

C

H

H

H

H

AldehydesAn aldehyde’s name ends in –alIt always has the C=O bond on thefirst carbon of the chain so it doesnot need an extra number. It is bydefault number one on the chain

Ethanal

C C C

O

H

H

H

H

H

H

Propanone

Ketones

Ketones end in -one

When ketones have 5C’s or morein a chain then it needs a numberto show the position of the doublebond. E.g. pentan-2-one

CH3 C CO

OHO

2-oxopropanoic acidCarboxylic acidsThese have the ending -

oic acid but no number isnecessary for the acidgroup as it must always beat the end of the chain.The numbering alwaysstarts from the carboxylicacid end

C C

O

O HH

C

H

H

H

H

Propanoic acid

If there are carboxylic acid groups on both ends of thechain then it is called a - dioic acid

C C

O

OH

O

OH

Ethanedioic acid

Note the e in this name

In the common naming version if thechain lengths are the same an –N is notused

CH3CH2CH2 N CH3

CH3

If a tertiary amine similar rules apply, andeach alkyl side group is given an N

N,N-dimethylpropylamine (common name)N,N-dimethylpropan-1-amine (IUPAC name)

If there are two amine groups then nameas following N

H(CH2)6 N

H

H H It could also be named1,6-diaminohexane

hexane-1,6-diamine

The prefix oxo- should beused for compounds thatcontain a ketone group inaddition to a carboxylic acidor aldehyde

C C C

O

H

H

H

H

C

H

C

OH

H

H

Pentane-2,4-dione

If two ketone groups thendi is put before –one andan e is added to the stem

O O

pentanedial

If two aldehyde groupsthen di is put before –aland an e is added tothe stem.


3

Carboxylic acid derivatives

EstersEsters have two parts to their names

C C C

O

O C

H

H

H

H

H

H

H

HThe bit ending in –yl comes from the alcohol that hasformed it and is next to the single bonded oxygen.The bit ending in –anoate comes from the carboxylic acid.(This is the chain including the C=O bond) methylpropanoate

Acid Anhydrides

This is called ethanoicanhydride. It is ethanoicbecause it is two ethanoategroups joined together..

If the alkyl groups are ofdifferent lengths theneach one is namede.g. ethanoic propanoic

anhydride

CH3 C

O

O

CCH3

O

CH3 C

O

O

CCH2

O

CH3

Acyl Chlorides

add –oyl chloride to the stemname

CH3 C

O

Clethanoyl chloride

Amides

Add –amide to the stemname CH3 C

O

NH2

ethanamide

Secondary and tertiary amidesare named differently to show thetwo (or three) carbon chains.The smaller alkyl group ispreceded by an –N which playsthe same role as a number inpositioning a side alkyl chain

CH3 CH2 C

O

NH CH3

N-methylpropanamide

CH3 CH2 C

O

N

CH3

CH3

N,N-dimethylpropanamideCH3 CH C

O

N

CH3

CH3

CH3

N,N,2-trimethylpropanamide

CCH

CH3

CH3 O

Cl

2-methylpropanoyl chloride

NitrilesThese end in –nitrile, but the C of the

CN group counts as the first carbon ofthe chain. Note the stem of the name isdifferent : butanenitrile and notbutannitrile.

C C

H

H

H

H

H

C

H

CN

H

butanenitrile

CH3C

CH3

OH

CN

2-hydroxy-2-methylpropanenitrile

C (CH2)3

O

C

O

Cl ClPentanedioyl dichloride


Structural isomers: same molecular formula differentstructures (or structural formulae)

Functional group isomers: Compounds with the same molecular formula but with atoms arranges to give differentfunctional groups

IsomersStructural isomerism can arise from•Chain isomerism•Position isomerism•Functional group isomerism

Stereoisomerism

Definition: Stereoisomers have the same structural formulaebut have a different spatial arrangement of atoms

There are two types of stereoisomerism:geometrical (E- Z isomerism) and opticalisomerism

Aldehydes and ketones of the same chain length would be classed as functionalgroup isomers- e.g. Propanal and propanone (both C3H6O)

4

See 3.1 for more on structural isomerism

CH3 CH2

CH2 C

O

OHCH3 C

O

O CH2 CH3 CH3 C

O

CH2 CH2

OH

CH2 CH CH CH2

OHOHQuestions often involve functional group isomers of carboxylic acids: including esters etc

Optical Isomerism

Optical isomerism occurs in carbon compounds with 4different groups of atoms attached to a carbon (calledan asymmetric carbon). C

H

C

H

C

H

H

H

C

H H

H

O

H

H

A carbon atom that hasfour different groupsattached is called a chiral(asymmetric) carbonatom

These four groups are arrangedtetrahedrally around the carbon.

CH5C2

CH3H

OH

CC2H5

CH3H

OH This causes twodifferent isomers thatare not superimposableto be formed. They aremirror images

Two compounds that are optical isomers ofeach other are called enantiomers.

A mixture containing a 50/50 mixture of thetwo isomers (enantiomers) is described asbeing a racemate or racemic mixture.

Many naturally occurring moleculescontain chiral C atoms, but are usuallyfound in nature as a pure enantiomer

Optical isomers have similar physical and chemical properties,but they rotate plane polarised light in different directions.

One enantiomer rotates it in one direction and the other enantiomerrotates it by the same amount in the opposite direction.

Different systems of nomenclature are isexistence for optical isomers. D/L or +/- arecommonly used, but both have beensuperseded by the more useful and informativeR/S system (this is not on the syllabus – forinformation only).

One optical isomer will rotate light clockwise (+)(calleddextrorotatory). The other will rotate it anticlockwise(-)(calledlaevorotatory).

-ve enantiomerAnticlockwiserotation

+ve enantiomerclockwiserotation

Racemateno rotationA racemic mixture (a mixture of equal amounts of the two

optical isomers) will not rotate plane-polarised light.

See 3.1 for EZ isomerism


C

C

C

C

C

C

C

C

O

O

N C

C

C C

C

O

ONH

H

H

H

H HH

HH

H

Chemical Reactions and Optical Isomers

Formation of a racemate

A racemate will be formed in a reactionmechanism when a reactant or intermediatehas a trigonal planar group in the moleculeis approached from both sides by an attackingspecies

:CN--NC:C

CH3

O

H

C

NC CH3

OH

H

C

CH3 CN

OH

H

There is an equal chance ofeither enantiomer forming soa racemate forms. Nooptical activity is seen

Major product s 90%

Minorproduct10%

If the alkene isunsymmetrical, addition ofhydrogen bromide canlead to isomeric products.

A racemate can also be formed in the AS reaction of theelectrophilic addition of HBr to an unsymmetrical alkene

CH2 CH CH2 CH3

H BrCH3

C+

CH2

CH3

H

C+

C CH2 CH3

H

HH

H

C

CH3

Br

H

CH2CH3

C

CH3

Br

H

CH2CH3

:Br -

:Br -

The bromide canattack this planarcarbocation fromboth sides leading toa racemate

CH2 CH2 CH2 CH3

Br

δ+ δ-

Drug action and optical isomersDrug action may be determined by the stereochemistry of the molecule.Different optical isomers may have very different effects

Chiral carbon

Thalidomide

One enantiomer of thalidomide causes birth defects inunborn children whilst the other had useful sedativeproblems. Unfortunately it was given in a racemicmixture when first used.

CH

C

CH

CH

C

CH

CH2

CH

CH3

CH3

CH

CH3

C

OH

O

Ibuprofen

5

CH3C

C2H5

ONucleophilic addition of HCN to

aldehydes and ketones (unsymmetrical)when the trigonal planar carbonylgroup is approached from both sides bythe HCN attacking species: results in theformation of a racemate

Mechanism for the reaction (drawn the same for both enantiomers)

:CN-

δ+

δ-

: H+-

C

CN

CH3 C2H5

O

C

CN

CH3 C2H5

O H


Documents

3.7 Organic naming and Isomerism continued