Sub-index for this page on the isomers of
C5H10O2
Introduction to isomerism for molecular formula C5H10O2
(a) 4 carboxylic acid isomers of formula
C5H10O2
(b) 9 ester isomers of
formula C5H10O2
(c)
Selected methoxy or hydroxy-aldehydes and methoxy or hydroxy-ketones of formula C5H10O2
(d) Selected ene-diols with molecular formula C5H10O2
(e) Selected alicyclic isomers of
formula C5H10O2
(f) Selected heterocyclic isomers of
formula C5H10O2
(g) A few extra comments on the spectra of selected isomers of C5H10O2
Introduction to the isomerism, selected constitutional isomers and stereoisomers of molecular formula C5H10O2
Composition of C5H10O2
Percent composition based on atomic masses C= 12.01 H = 1.01 O = 16.00 and Mr(C5H10O2) = 102.15
Element composition (to two dp): carbon = 58.79% hydrogen = 9.89% oxygen = 31.33%
Empirical formula = molecular formula = C5H10O2
Please note there are a very large number of isomers of molecular formula C5H10O2 and this page presents a selection of them and where appropriate indicates the type of isomerism involved.
Structural isomerism – isomers of the same specific molecular formula, based on different connectivity’s of the constituent atoms (the constitutional isomers), so they cannot be spatially identical (but sometimes can be defined as having the same shape).
This includes (a) carbon chain variation (usually need a minimum of 4 atoms), (b) change in position of a substituent or functional group and (c) functional group isomerism where the atoms have a different connectivity configuration, usually with significant differences in chemical and physical properties e.g.
In terms of isomers of C5H10O2 there are
(a) chain variations based on the C or O atoms, open chain aliphatic linear, branched and cyclic (alicyclic) and heterocyclic structures,
(b) positional isomers e.g. hydroxy group in the aldehydes and ketones, substituent positions in the cyclic compounds.
(c) They are all functional group isomers of each other e.g. in terms e.g. carboxylic acids, esters, hydroxy-aldehydes, hydroxy-ketones, alkene-alcohols (unsaturated ‘enols’), saturated substituted cycloalkane based molecules.
Stereoisomerism – isomers based on the same connectivity of the atoms (same constitutional formula), but in some way, they are 2D or 3D spatially different non-superimposable images (e.g. E/Z ‘geometrical’ isomers or mirror image R/S ‘optical‘ isomers)
This is where molecules have the same basic constitutional structural formula, but isomers differ in the 2D/3D arrangement of the atoms.
For stereoisomers, the (CIP) abbreviation means the IUPAC Cahn-Ingold-Prelog priority order rule for assigning E/Z (geometrical) and R/S (optical) stereoisomers.
E/Z stereoisomerism was called ‘geometrical isomerism’ e.g. cis (= Z) and trans (= E) isomers of alkenes or disubstituted cyclic alkanes where there are 2D/3D spatial variations that are not mirror images and not super imposable.
There are many examples of E/Z geometrical isomers in open chain compounds via the C=C restricted rotation or two substituents in a ring where both can be above the ‘plane’ of a ring or one above and one below the plane of the ring – can be complicated with overlapping R/S optical isomerism for the same molecule as in the case 1,2-dimethylpropane.
R/S stereoisomerism was called ‘optical isomerism’, the pairs of isomers are called enantiomers, which are 3D non-superimposable mirror image forms of the molecule (enantiomers).
The molecule must have a chiral centre (a stereocentre), that is an asymmetric carbon atom with four different atoms/groups attached to it.
There many examples of R/S optical isomers, but, as already mentioned, it can be complicated with overlapping E/Z geometric isomers for the same molecule and some molecules, particularly the ‘cyclic’ ring containing molecules may have two or three chiral centres (asymmetric carbon atoms).
NOTE
Initially I have chosen examples of isomers that pre-university students are most likely to come across, and that is usually carboxylic acids and esters.
There are huge number of constitutional isomers (I’ve identified at least ~50, theoretical or actual) and many stereoisomers (at least~29 pairs) of formula C5H10O2 and there are many more.
NEXT (a-e) Details of selected constitutional isomers and stereoisomers of C5H10O2 below.
(a) and (b) Examples of aliphatic carboxylic acid and ester structural isomers of molecular formula C5H10O2
(a) There are four constitutional isomeric carboxylic acids (1) to (4) of molecular formula C5H10O2
Note that COOH is shorthand for the 4 carboxylic acid’s functional group O=C-O-H
(1) pentanoic acid , , ,
Number of low resolution NMR chemical shift δ signal peaks: 5 1H and 5 13C (email if disagree?)
1H NMR ratio of peaks: 3 : 2 : 2 : 2 : 1 (for equivalent protons)
(2) 2-methylbutanoic acid , CH3CH2CH(CH3)COOH, ,
This molecule has a chiral carbon (the C of the CH group), so this molecule will exhibit R/S stereoisomerism, optical isomers – non-superimposable mirror image forms known as enantiomers.
Number of low resolution NMR chemical shift δ signal peaks: 5 1H and 5 13C (email if disagree?)
1H NMR ratio of peaks: 3 : 2 : 1 : 3 : 1 (for equivalent protons)
(3) 3-methylbutanoic acid , (CH3)2CHCH2COOH, ,
Number of low resolution NMR chemical shift δ signal peaks: 4 1H and 4 13C (email if disagree?)
1H NMR ratio of peaks: 6 (3+3) : 1 : 1 (for equivalent protons)
(4) 2,2-dimethylpropanoic acid , (CH3)3CCOOH , ,
Number of low resolution NMR chemical shift δ signal peaks: 2 1H and 3 13C (email if disagree?)
1H NMR ratio of peaks: 9 (3×3) : 1 (for equivalent protons)
(b) There are nine constitutional isomeric esters e.g (5) to (13) of molecular formula C5H10O2
Note that COOC shorthand for the ester linkage functional group O=C-O-C
The 9 esters illustrated below.
(5) methyl butanoate (methyl butyrate), ,
An ester made from butanoic acid and methanol.
(6) ethyl propanoate, ,
An ester made from propanoic acid and ethanol.
(7) propyl ethanoate , ,
An ester made from ethanoic acid and propan-1-ol.
Brief notes on the IR, mass, 1H NMR & 13C NMR spectrums of propyl ethanoate
(8) butyl methanoate, HCOOCH2CH2CH2CH3 , ,
An ester made from methanoic acid and butan-1-ol.
(9) 1-methylethyl ethanoate, CH3COOCH(CH3)2 , ,
An ester made from ethanoic acid and propan-2-ol. (isopropyl acetate)
(10) methyl 2-methylpropanoate, (CH3)2CHCOOCH3 , ,
An ester made from 2-methylpropanoic acid and methanol.
(11) 1-methypropyl methanoate, HCOOCH(CH3)CH2CH3 , ,
An ester made from methanoic acid and butan-2-ol, R/S isomers
(12) 2-methylpropyl methanoate, HCOOCH2CH(CH3)2 , ,
An ester made from methanoic acid and 2-methylpropan-1-ol
(13) 1,1-dimethylethyl methanoate, HCOOC(CH3)3 , ,
An ester made from methanoic acid and 2-methylpropan-2-ol
For examples (c) onwards, you are more likely to come across them at university level, but the odd structure may crop up in advanced pre-university chemistry courses.
(c) There are also lots of methoxy- or hydroxy-aldehydes and methoxy- or hydroxy-ketones with a C5H10O2 molecular formula
For the functional groups in abbreviated structural formulae, note …
CO is the >C=O carbonyl group of ketones (named …one),
and CHO is the H-C=O aldehyde group (named …al),
plus some have an ether C-O-C group linkages (instead of an OH hydroxy group) e.g. via the methoxy group OCH3
A molecule with a chiral carbon (often the C of a CH group), will exhibit R/S stereoisomerism, exhibiting R/S isomers, optical isomers – non-superimposable mirror image forms known as enantiomers.
Note that IUPAC nomenclature rules state that the aldehyde or ketone group takes precedence over the ether methoxy or hydroxy alcohol groups, the compounds are named with the …al or …one suffix and the alcohol named as the hydroxy substituent prefix. Take care with the prefix number too!
Some of the hydroxy-aldehyde or methoxy-aldehyde isomers of molecular formula C5H10O2
Isomers with a linear carbon chain with an aldehyde and alcohol functional groups
CH3CH2CH2CH(OH)CHO 2-hydroxypentanal, R/S isomers, C2 chiral
CH3CH2CH(OH)CH2CHO 3-hydroxypentanal, R/S isomers C3 chiral
CH3CH(OH)CH2CH2CHO 4-hydroxypentanal, R/S isomers C4 chiral
HOCH2CH2CH2CH2CHO 5-hydroxypentanal
and two methoxy-aldehydes (ether-aldehydes)
CH3OCH2CH2CH2CHO 4-methoxybutanal
CH3CH(OCH3)CH2CHO 3-methoxybutanal, R/S isomers C3 chiral
Isomers with a branched carbon chain
(CH3)2CHCH(OH)CHO 2-hydroxy-3-methylbutanal, R/S isomers C2 chiral
(CH3)2C(OH)CH2CHO 3-hydroxy-3-methylbutanal
HOCH2CH(CH3)CH2CHO 4-hydroxy-3-methylbutanal, R/S isomers C3 chiral
Some of the hydroxy-ketone or methoxy-ketone isomers of molecular formula C5H10O2
Isomers with a linear carbon chain with a ketone and alcohol functional group
CH3CH2CH2COCH2OH 1-hydroxypentan-2-one (1-hydroxy-2-pentanone)
CH3CH2CH(OH)COCH3 3-hydroxypentan-2-one (3-hydroxy-2-pentanone), R/S isomers, C3 chiral
CH3CH(OH)CH2COCH3 4-hydroxypentan-2-one (4-hydroxy-2-pentanone), R/S isomers, C4 chiral
HOCH2CH2CH2COCH3 5-hydroxypentan-2-one (5-hydroxy-2-pentanone)
HOCH2CH2COCH2CH3 1-hydroxypentan-3-one (1-hydroxy-3-pentanone)
CH3CH(OH)COCH2CH3 2-hydroxypentan-3-one (2-hydroxy-3-pentanone), R/S isomers, C3 chiral
and two methoxy-ketones (ether-ketones)
CH3OCH2COCH2CH3 1-methoxybutan-2-one (1-methoxy-2-buttanone)
CH3CH(OCH3)COCH3 3-methoxybutan-2-one (3-methoxy-2-butanone), R/S isomers, C4 chiral
Isomers with a branched carbon chain
HOCH2COCH(CH3)2 1-hydroxy-3-methylbutan-2-one
CH3COC(OH)(CH3)2 3-hydroxy-3-methylbutan-2-one
CH3COCH(CH3)CH2OH 4-hydroxy-3-methylbutan-2-one, R/S isomers, C3 chiral
