Cis-Trans Isomerism In Organic Compounds For CAPE Unit 2 Chemistry Students

CIS‐TRANSCIS TRANS

ISOMERISMISOMERISM

IN ORGANIC COMPOUNDSIN ORGANIC COMPOUNDS

FOR

CAPE UNIT 2 STUDENTS

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What Are Geometrical Isomers

Geometrical isomers are isomers which have th l l f l d ththe same molecular formula and the same molecular structure. In other words, they have the same atoms and the same bonds. However, the isomers are different because some of the atoms are arranged differently in space, and the isomers cannot bespace, and the isomers cannot be interconverted without breaking and remaking covalent bonds

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It’s Not Confined To Alkenes

For CAPE, cis‐trans isomerism is confined to , fcompounds containing the carbon to carbon double bond (i e alkenes and theirdouble bond (i.e. alkenes and their derivatives).

Cis‐trans isomers however, also occur in transition metal complexes, and compoundstransition metal complexes, and compounds containing only single carbon to carbon bonds.

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How Does This Arise

Alkenes having two different substituents at h d f th d bl b d i t teach end of the double bond can exist as two

configurational isomers. Simple alkenes can be defined as cis or trans depending on whether substituents at different ends of the alkene are on the same side of the alkene (i.e. cis) or on opposite sides (i.e. trans).on opposite sides (i.e. trans).


What Is The ‘Same Side’

Groups on the same side of the double bond f i t th id ffor cis‐trans purposes are on the same side of an imaginary line that runs through the nuclei of the double bonded carbon atoms.

Methyl substituents on the ‘same side’.

Hydrogen atoms on the ‘same side’


Hydrogen atoms on the same side .

Other Names For Cis‐trans Isomers

1 G t i ( l) i1. Geometric(al) isomers.

2 Configurational isomers2. Configurational isomers.

3. Stereoisomers.


Substituents

These are groups or atoms that have replaced (substituted) atoms of an original compound.( ) g p

Hence the ethyl group in the compound on the following slide can be viewed as a substituent on ethene, since it substituted itself for one hydrogen atom of the original compound.compound.


Ethyl substituent on an ethene molecule.

In spotting cis‐trans isomers, it helps to draw d if h b i dcompounds as if they were substituted

ethene molecules, as above.


After deciding if they are cis‐ or trans‐ the molecules must be named according to the longest continuous chain of carbon atoms, as will be shown in the examples which follow.

Hence the compound seen on the previous slide is but 1 ene (Notice that its name isslide is but‐1‐ene. (Notice that its name is not a derivative of ethene since the longest

b h i i h bcarbon chain contains more than two carbon atoms.)


Requirements For Cis‐Trans Isomerism

1 Th h ld b d bl b d i th1. There should be a double bond in the molecule.

2. The two atoms or groups attached to each doubly bonded carbon atom should bedoubly bonded carbon atom should be different.


Cis‐Trans Isomerism Cannot Occur

1 Wh f th d bl b d d t1. When one of the doubly bonded atoms carries two identical groups.

2. When the alkene is tri‐ or tetra substitutedtetra‐substituted.

In the case of tri‐ or tetra‐substituted alkenes,In the case of tri or tetra substituted alkenes, the E‐Z system of nomenclature replaces cis‐trans

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The Good News

For CAPE, then, since E‐Z nomenclature is not required knowledge, spotting cis‐ and trans‐q g , p gisomers is easy.

This is because there will always be two hydrogen atoms among the four groups y g g g pattached to the doubly bonded carbon if a compound can possibly exhibit this type ofcompound can possibly exhibit this type of isomerism.


How To Spot The Isomers For CAPE

•If one of the doubly bonded carbon atoms has two hydrogen atoms, cis‐trans isomerism y g ,does not occur.

•If both hydrogen atoms are on different carbons cis‐trans isomers occur. The cis‐isomer is the one with the two hydrogen atoms on the same side of the double bond.atoms on the same side of the double bond. The trans‐ isomer is the other.


Consider C4H8

This compound has four isomers which we willlook at one by one.y


Isomer #1 – but‐1‐ene

Also called 1‐butene, this compound has no pcis‐trans isomers. This is due to the two identical hydrogen atoms on one of the doublyidentical hydrogen atoms on one of the doubly bonded carbon atoms.


Notice that when we are trying to figure out if there is cis‐trans isomerism the double bondthere is cis‐trans isomerism, the double bond is written at the ‘center’ of the molecule, and f t f th t bfour groups come out of the two carbon atoms diagonally.

The compound is not written as a straightThe compound is not written as a straight chain (or straight with perpendicular

)branches) because this makes cis‐trans isomers harder to notice.


Isomer #2 – 2‐methylpropene

No cis‐trans isomerism is possible with this compound, since it has identical groups on BOTH of the doubly bonded carbon atoms.


Isomer #3 – cis‐but‐2‐ene

Also called cis‐2‐butene, it is the cis‐ isomer because the two –CH3 substituents are on the same side of the molecule.


Isomer #4 – trans‐but‐2‐ene

Also called trans‐2‐butene, it is the trans‐isomer because the two –CH3 substituents are on opposite sides of the molecule.


Are They The Same Compound?

Ci d t i id d t bCis‐ and trans‐ isomers are considered to be different compounds.

•The cis‐isomer is more polar than the trans‐isomer and has a higher boiling pointisomer and has a higher boiling point.

•The trans‐ isomer has a higher melting pointThe trans isomer has a higher melting point and is more stable.


Try Some CAPE Past Paper Questions

C t t if th i i t tiContact me if there is any cis‐trans question causing you difficulty, and I will try my best to help ASAP.


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Cis-Trans Isomerism In Organic Compounds For CAPE Unit 2 Chemistry Students