Organic Naming and Isomerism Revision Guide PDF

3.7 Organic naming and Isomerism continued See chapter 3.1 for basic naming of organic molecules. This chapter extends the naming for functional groups met in next few chapters prefix / suffix homologous functional group...

3.7 Organic naming and Isomerism continued See chapter 3.1 for basic naming of organic molecules. This chapter extends the naming for functional groups met in next few chapters prefix / suffix homologous functional group example (* = usual use) series O suffix -al H O aldehydes C H prefix formyl- H C C H ethanal H O suffix* -one H O H ketones prefix oxo- C H C C C H Propanone H H O suffix -oic acid H O carboxylic acids C OH H C C OH Ethanoic acid H suffix -nitrile H H nitriles prefix cyano- C N H C C C N Propanenitrile H H suffix* -amine amines C NH2 prefix amino- H H H Propylamine H C C C NH2 Or propan-1-amine H H H H O H O esters C O -yl –oate H C C O C H methylethanoate H H O O acyl chloride C -oyl chloride CH 3 C ethanoylchloride Cl Cl O O amide C -amide CH3 C ethanamide NH2 NH2 -oic anhydride acid anhydrides O O R C CH3 C O O Ethanoic anhydride R C CH3 C O O When compounds contain more than one functional group, the order of precedence determines which groups are named with prefix or suffix forms. The highest precedence group takes the suffix (and the lowest number on the carbon chain), 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 N Goalby chemrevise.org 1  Aldehydes H O Ketones H O H An aldehyde’s name ends in –al It always has the C=O bond on the Ketones end in -one H C C C H first carbon of the chain so it does H C C When ketones have 5C’s or more not need an extra number. It is by H H in a chain then it needs a number default number one on the chain propanone H H to show the position of the double bond. E.g. pentan-2-one ethanal H O H O H If two ketone groups then di is put before –one and H C C C C C H If two aldehyde groups an e is added to the stem H H H then di is put before –al O O and an e is added to pentane-2,4-dione the stem. pentanedial O The prefix oxo- should be used for compounds that H3C C C contain a ketone group in O OH addition to a carboxylic acid or aldehyde 2-oxopropanoic acid Carboxylic acids These have the ending - oic acid but no number is H H O If there are carboxylic acid groups on both ends of the necessary for the acid chain then it is called a - dioic acid group as it must always be H C C C O O at the end of the chain. Ethanedioic acid The numbering always C C O H starts from the carboxylic H H HO OH Note the e in this name acid end Propanoic acid Amines H H H H2N O These end in –amine. There is, however, rather confusingly two H C C C NH2 If there is another priority CH C ways of using this suffix. functional group as well The exam board tend to use the common H H H as the amine group then H3C O H version where the name stem ends in -yl propylamine the prefix amino is used. propylamine. Or propan-1-amine 2-aminopropanoic acid. Another version of the same chemical is propan-1-amine. (This is used in the same way as naming alcohols) If the amine is secondary and has two alkyl CH3CH2CH2NHCH3 groups attached to the nitrogen, then each chain N-methylpropylamine (common name) is named and the smaller alkyl group is preceded N-methylpropan-1-amine (other name) by an –N which plays the same role as a number in positioning a side alkyl chain CH2 CH2 H3C NH CH3 In the common naming version if the diethylamine (common name- does not use N if chains are chain lengths are the same an –N is not same length) used N-ethylethanamine (other name does still use N) If a tertiary amine similar rules apply, and CH3CH2 CH2 N CH3 each alkyl side group is given an N CH3 N,N-dimethylpropylamine (common name) N,N-dimethylpropan-1-amine (other name) If there are two amine groups then name H H as following N (CH2)6 N It could also be named 1,6-diaminohexane H H hexane-1,6-diamine N Goalby chemrevise.org 2  Nitriles H H H OH H3C These end in –nitrile, but the C of the C H C C C CN CN group counts as the first carbon of C the chain. Note the stem of the name is H3C N H H H different : butanenitrile and not 2-hydroxy-2-methylpropanenitrile butannitrile. butanenitrile Carboxylic acid derivatives Esters O Esters have two parts to their names H 3C CH 2 C O CH 3 The bit ending in –yl comes from the alcohol that has formed it and is next to the single bonded oxygen. The bit ending in –anoate comes methylpropanoate from the carboxylic acid. (This is the chain including the C=O bond) O O O O ethylbenzoate Ethyl 3-methylbutanoate Acid Anhydrides O O If the alkyl groups are of This is called ethanoic CH3 C different lengths then CH3 C anhydride. It is ethanoic O each one is named because it is two ethanoate O e.g. ethanoic propanoic groups joined together. CH3 C anhydride CH3 CH2 C. O O Acyl Chlorides O H3C O O O add –oyl chloride to the stem CH3 C CH C C (CH2)3 C name H3C Cl Cl Cl Cl ethanoyl chloride 2-methylpropanoyl chloride pentanedioyl dichloride Amides O Secondary and tertiary amides Add –amide to the stem are named differently to show the O name CH3 C two (or three) carbon chains. NH2 The smaller alkyl group is H3C CH2 C NH CH3 preceded by an –N which plays ethanamide the same role as a number in N-methylpropanamide positioning a side alkyl chain O CH3 CH3 O CH3 H3C CH2 C N CH3 H3C CH C N CH3 N,N-dimethylpropanamide N,N,2-trimethylpropanamide N Goalby chemrevise.org 3  Isomers Structural isomerism can arise from Structural isomers: same molecular formula different structures (or structural formulae) Chain isomerism Position isomerism See 3.1 for more on structural isomerism Functional group isomerism Functional group isomers: Compounds with the same molecular formula but with atoms arranges to give different functional groups Aldehydes and ketones of the same chain length would be classed as functional group isomers- e.g. Propanal and propanone (both C3H6O) Questions often involve functional group isomers of carboxylic acids: including esters etc H 3C CH2 O O O OH OH OH CH 2 C H 3C C O CH 2 CH 3 C CH 2 CH 2 H 2C CH CH CH 2 H 3C OH Stereoisomerism Definition: Stereoisomers have the same structural formulae There are two types of stereoisomerism: but have a different spatial arrangement of atoms geometrical (E- Z isomerism) and optical isomerism See 3.1 for EZ isomerism Optical Isomerism A carbon atom that has Optical isomerism occurs in carbon compounds with 4 H H H H four different groups different groups of atoms attached to a carbon (called H C C C C H attached is called a chiral an asymmetric carbon). (asymmetric) carbon H H O H atom These four groups are arranged H tetrahedrally around the carbon. OH OH This causes two A mixture containing a 50/50 mixture of the different isomers that C C two isomers (enantiomers) is described as are not superimposable CH3 H3C being a racemate or racemic mixture. H5 C2 C2 H5 to be formed. They are H H mirror images Many naturally occurring molecules contain chiral C atoms, but are usually Two compounds that are optical isomers of found in nature as a pure enantiomer each other are called enantiomers. Different systems of nomenclature are is Optical isomers have similar physical and chemical properties, existence for optical isomers. D/L or +/- are but they rotate plane polarised light in different directions. commonly used, but both have been superseded by the more useful and informative One enantiomer rotates it in one direction and the other enantiomer R/S system (this is not on the syllabus – for rotates it by the same amount in the opposite direction. information only). One optical isomer will rotate light clockwise (+)(called dextrorotatory). The other will rotate it anticlockwise(-)(called laevorotatory). Racemate A racemic mixture (a mixture of equal amounts of the two -ve enantiomer +ve enantiomer no rotation optical isomers) will not rotate plane-polarised light. Anticlockwise clockwise rotation rotation N Goalby chemrevise.org 4 Chemical Reactions and Optical Isomers Formation of a racemate CH3 A racemate will be formed in a reaction H mechanism when a reactant or intermediate C -NC: has a trigonal planar group in the molecule :CN- is approached from both sides by an attacking species O H H H 3C CN NC CH3 C There is an equal chance of C either enantiomer forming so a racemate forms. No OH OH optical activity is seen Mechanism for the reaction (drawn the same for both enantiomers) Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) O δ- O: - H+ O H when the trigonal planar carbonyl group + is approached equally from both sides by Cδ H3C C C 2H 5 H3C C C 2H 5 the CN- attacking species: results in the H3C C2H5 formation of a racemate (equal amounts of both enantiomers) :CN- CN CN A racemate can also be formed in the reaction of the electrophilic addition of HBr to an unsymmetrical alkene H H :Br - The bromide can H + attack this planar C CH2 C δ+ δ- carbocation from C H Br both sides leading to CH2 Br CH2 H3C CH3 H3C Br a racemate. CH3 CH3 CH3 H2C CH CH2 CH3 :Br - Major product s 90% If the alkene is H H unsymmetrical, addition of + C C CH2 CH3 hydrogen bromide can CH2 CH2 CH2 CH3 Minor lead to isomeric products. H H product Br 10% Drug action and optical isomers Drug action may be determined by the stereochemistry of the molecule. Different optical isomers may have very different effects Thalidomide Ibuprofen H CH3 H O C H H H3C HC CH CH CH3 C C H C C C H CH2 C C HC C N H C C C C C H O CH CH C O C N H O O HO H Chiral carbon One enantiomer of thalidomide causes birth defects in unborn children whilst the other had useful sedative problems. Unfortunately it was given in a racemic mixture when first used. N Goalby chemrevise.org 5