All Advanced Organic Chemistry Notes

Some physical similarities e.g. low boiling colourless polar liquids or gases, (1) and (2) also show chemical similarities, as do (3) and (4), but there are significant chemical differences between all four shown below.

(1) , propanal (an aldehyde), bpt 49^oC, adds HCN to give hydroxynitrile, gives yellow-orange ppt with 24DNPH, produces the primary alcohol, propan-1-ol, on reduction, readily oxidised to propanoic acid, gives silver mirror with ammoniacal silver nitrate and red-brown ppt with Fehlings/Benedict's reagent. I2 reaction?

(2) , propanone (a ketone), bpt 56^oC,  adds HCN to give hydroxynitrile, gives yellow-orange ppt with 24DNPH, produces secondary alcohol, propan-2-ol, on reduction, NOT readily oxidised, NO silver mirror with ammoniacal silver nitrate and NO red-brown ppt with Fehlings/Benedict's reagent. I2 reaction?

(3) , prop-2-ene-1-ol (a bi-functional molecule alkene/alcohol or enol), bpt 97^oC, higher bpt due to hydrogen bonding via -OH (not possible with 1 and 2 above), gives electrophilic addition reaction of Br₂, H₂O, HI etc. like any other alkene, reacts with sodium to give H₂ and forms esters with carboxylic acids or acid chlorides just like alcohols do, NO reaction with ammoniacal silver nitrate, Fehlings/Benedict's reagent or 24DNPH.

(4)  , cyclopropanol (an alicyclic secondary alcohol, bpt ?, very unstable, difficult to study, and readily isomerises to (1) propanal (see case study 1c.4 below.

Theoretically has the chemistry of a secondary alcohol e.g. oxidised to the ketone cyclopropanone, reacts with sodium to give H₂, forms esters with carboxylic acids or acid chlorides.

(5) , methoxyethene (a bi-functional ether-alkene), bpt 5^oC, gives electrophilic addition reaction of Br₂, H₂O, HI etc. like any other alkene, but no aldehyde, ketone or alcohol chemistry.

(6) , 1,2-epoxypropane (a cyclic-ether), bpt 35^oC, no alkene, aldehyde, ketone or alcohol chemistry.

(7) , 1,3-epoxypropane (a cyclic-ether), bpt 49^oC,  no alkene, aldehyde, ketone or alcohol chemistry.

 

Again, for a 'small' molecular formula, C₃H₆O₂ packs quite an isomeric punch! but don't worry too much, (1) to (2) are the ones whose detailed structure, naming, physical properties and chemical reactions you should be very familiar with.

(3) to (5) you should cope with in a functional group concept Q and (6) to (7) I wouldn't worry too much about at all!

14.1.2(c) Structural Isomerism - Functional Group isomerism

Case study 1c.4 The functional group isomerisation reactions of cyclopropane or cyclopropanol

In these cases one isomers is changed into another, with a different functional group, without any other reactants or products, plus the obvious differences in functional group chemistry.

Cyclopropane is quite unstable because the ring is very strained due to the enforced geometry, i.e. the C-C-C bond angle of 60^o, rather than the usual 'tetrahedral' bond system producing C-C-C angles of 109^o.

On heating or catalysis, isomerization occurs and cyclopropane (a cyclo-alkane, alicyclic) is readily converted to the much more stable propene (linear alkene).

(1) (2)

Unlike cyclopropane, propene will undergo all the additions reactions of alkenes e.g. addition of halogens and halogen halides to form saturated haloalkane molecules.

Similarly, unstable molecule (3) cyclopropanol (a alicyclic secondary alcohol) readily isomerizes to form the more stable (4) propanal (an aldehyde).

(3) (4)

Again, you have significant differences in chemistry e.g. cyclopropanol will form esters on reaction with carboxylic acids which propanal cannot do. Conversely, propanal undergoes addition reactions with e.g. hydrogen cyanide which cyclopropanol cannot do.

14.1.2(c) Structural Isomerism - Functional Group Isomerism

Case study 1c.5 An alcohol, phenols and ether based on C₇H₈O (aromatic compounds)

These are all colourless liquids but show great differences in chemical properties.

(1) , phenylmethanol (a aliphatic primary alcohol, OH NOT attached directly to benzene ring), OH NOT attached directly to benzene ring, mpt -25^oC, bpt 205^oC , it can be oxidised to an aldehyde, forms esters with carboxylic acids or acid chlorides, but can't act as ligand to form a purple complexes with the iron(III) ion.

(2) , methyl-3-phenol (a aromatic phenol, OH attached directly to benzene ring), mpt 12^oC, bpt 202^oC, forms esters with carboxylic acids or acid chlorides, but phenols can act as ligands and form purple complexes with the iron(III) ion.

As phenols, they form diazo dyes when coupled with diazonium salts.

There two other positional isomers, namely (3) methyl-2-phenol, mpt 31^oC, bpt 191^oC

and (4) methyl-4-phenol, mpt 35^oC, bpt 202^oC, not shown, but very similar physically and chemically to (2).

(5) , methoxybenzene (a mixed aliphatic/aromatic ether), mpt -37^oC, bpt 154^oC, it cannot be oxidised to an aldehyde, cannot form esters with carboxylic acids or acid chlorides, or purple complexes with the iron(III) ion.

The boiling point is relatively lower than the others because hydrogen bonding via O-H is not possible as it is in (1) and (2).

14.1.2(c) Structural Isomerism - Functional Group Isomerism

Case study 1c.6 Aromatic compounds based on C₇H₇NO₂ 

This molecular formula can give rise to many isomers of a wide variety of chemistry and a few examples are quoted below.

(1) , methyl-2-nitrobenzene (a tri-functional nitro, alkane (via -CH₃) and benzene ring), colourless liquid, mpt -3^oC, bpt 223^oC.

It has two other positional isomers, ...-3-... and ...-4-...

The melting point of (1) is significantly lower than (2) and (3) described below due to lack of H-bonding via the -OH in (2) or -CONH₂ group in (3).

Chemistry of (1) e.g.

(i) the nitro group can be reduced to an (-NH₂) amine by refluxing with Sn_(s)/HCl_(aq)

(ii) the -CH₃ can be 'free radical' chlorinated with Cl₂/uv light

(iii) the four 'vacant' C-H positions around the benzene ring can undergo electrophilic substitution (nitration, chlorination, sulfonation, alkylation, acylation etc.) - this applies to (2) and (3) too.

(2) , 3-aminobenzoic acid (a tri-functional primary amine-carboxylic acid and benzene ring), colourless solid, mpt 180^oC, bpt ?. It has two other positional isomers, 2-... and 4-...

Chemistry of (2) e.g.

(i) the -NH₂ can forms salts and diazotised to couple with phenols to make dyes

(ii) the -COOH group reacts with metals/carbonates to give salts + H₂/CO₂ gas respectively, and with alcohols to form esters

(iii) the benzene ring can undergo electrophilic substitution - this applies to (1) and (3) too.

(3) , 4-hydroxybenzamide (a tri-functional phenol-primary amide and benzene ring), colourless solid, mpt 162^oC, bpt ?. It has two other positional isomers, 2-... and 3-...

Chemistry of (3) e.g.

(i) complexes with Fe³⁺_(aq) via -OH phenol group

(ii) couples with diazotised aromatic amines to form dyes

(iii) the benzene ring can undergo electrophilic substitution - this applies to (1) and (2) too.

Apart from electrophilic substitution in the benzene ring, all three molecules have their own unique functional group chemistry in terms of at least two reactions.

 All Advanced Organic Chemistry Notes

 Index of sets of isomers for a given molecular formula, some include IR and NMR spectroscopy data

 The chemistry of AROMATIC COMPOUNDS - benzene and derivatives

What you need to know about functional group isomerism, functional group isomerism is defined, examples of functional group isomerism explained, defining what is meant by functional group isomerism, similarities and differences between the physical and chemical properties of the functional group isomers are described and explained, the structural formula, skeletal formula and IUPAC names are given for the functional group isomers