The 4 constitutional structural isomers of molecular formula C₄H₉F, C₄H₉Cl, C₄H₉Br or C₄H₉I

Relative molecular mass and percent composition based on atomic masses:

C 12.01,  H 1.01, F 19.00,  Cl 35.45,  Br 79.90, I 126.90

If applicable (see isomerism summary at the end of the page)

Structural isomerism  - isomers based on different connectivity's of the constituent atoms, so cannot be spatially identical (but 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.

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)

E/Z stereoisomerism was called 'geometrical isomerism' e.g. cis and trans isomers of alkenes or disubstituted cyclic alkanes where there are 3D spatial variations that are not mirror images and not super imposable.

Not applicable here.

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. The molecule must have a chiral centre (a stereocentre), that is an asymmetric carbon atom with four different atoms/groups attached to it.

The is one example of R/S optical isomerism, only one constituent isomer (per haloalkane formula) can form a pair of enantiomers.

Note

For images of the isomers of C₄H₉X, X = F, Cl, Br, I halogens of group 7/17 of the periodic table.

They all have a single halide functional group of a haloalkane i.e. C-X

The images of C₄H₉X (X = F, Cl, Br and I) presented are theoretical, in the sense that some may be so unstable as not to exist, but you will find most, and sometimes all of them, on the internet.

The are 4 constitutional-structural isomers of molecular formula C₄H₉X (X = F, Cl, Br and I), irrespective of any E/Z or R/S isomerism that may be possible.

Including the two R/S isomers, there are a total of 5 distinct unique isomers of C₄H₉X

They are all halogen substitution products of the hydrocarbon alkanes butane and methylpropane and are examples of the group of organic compounds known as haloalkanes or halogenoalkanes.

However, one structural isomer can exhibit R/S ('optical') isomerism, so there are actually a total of 5 distinct isomers for the halogen compound molecular formula C₄H₉X. E/Z isomerism is not possible for these compounds.

(1) CH₃CH₂CH₂CH₂X, abbreviated structural formula , , skeletal formula

1-fluorobutane, 1-chlorobutane, 1-bromobutane, 1-iodobutane

Primary haloalkanes

They cannot exhibit E/Z or R/S stereoisomerism and are primary haloalkanes.

 e.g. , , , primary haloalkane

structural formula, full displayed molecular structure and skeleton formula of 1-chlorobutane.

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 3 : 2 : 2 : 2  (for equivalent protons)

Index of ¹H NMR spectra organic compounds and Index of ¹³C NMR spectra organic compounds

 

(2) CH₃CH₂CHXCH₃, abbreviated structural formula , , skeletal formula

2-fluorobutane, 2-chlorobutane, 2-bromobutane, 2-iodobutane, they are secondary haloalkanes.

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 3 : 2 : 1 : 3  (for equivalent protons)

These molecules exhibit R/S stereoisomerism, diagrams of enantiomers are shown below.

, CH₃CH₂CHXCH₃ chiral carbon C

Carbon atom 2 is a chiral (asymmetric) with four different groups attached to it.

e.g. , , , secondary haloalkane

structural formula, full displayed molecular structure and skeleton formula of 2-chlorobutane.

, CH₃CH₂CHClCH₃ chiral carbon C

So the full IUPAC name would be e.g. (R)-2-chlorobutane  and  (S)-2-chlorobutane

From the CIP assignment priority rule for R/S isomers of all four formulae:

_ZX  >  ₆C₆C  > ₆C  > ₁H

based on the groups connected to the chiral carbon, X = halogen, and Z = 9, 17, 35 or 53)

These R/S assignments are based on the Cahn-Ingold-Prelog priority rules.

 

(3) (CH₃)₂CHCH₂X, abbreviated structural formula , , skeletal formula

1-fluoro-2-methylpropane, 1-chloro-2-methylpropane, 1-bromo-2-methylpropane, 1-iodo-2-methylpropane

They cannot exhibit E/Z or R/S stereoisomerism. They are primary haloalkanes.

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 6 (3+3) : 1 : 2  (for equivalent protons)

e.g. , , , primary haloalkane

structural formula, full displayed molecular structure & skeleton formula of 1-chloro-2-methylpropane

(3) and (4) have a branched carbon chain.

 

(4) (CH₃)₃CX, abbreviated structural formula , , skeletal formula

2-fluoro-2-methylpropane, 2-chloro-2-methylpropane, 2-bromo-2-methylpropane, 2-iodo-2-methylpropane

They cannot exhibit E/Z or R/S stereoisomerism. They are tertiary haloalkanes.

Number of low resolution NMR chemical shift δ signal peaks: 1 ¹H and 2 ¹³C (email if disagree?)

, , , 2-chloro-2-methylpropane, tertiary haloalkane

structural formula, full displayed molecular structure & skeleton formula of 2-chloro-2-methylpropane

Note how spectra can be used to distinguish which halogen compound and which isomer

Index of revision notes on the chemistry ALKENES including reactions and polymers

 Advanced Level pre-university organic chemistry notes