Alkenes & Stereoisomerism - Organic Chem (7.5)

Question 1

Describe the bonding in alkanes

Answer 1

In alkanes:

• C forms 4 bonds to other atoms
• Angles btw. bonds is 109.5 to minimise repulsion & the shape of each C centre is tetrahedral
• C-C single bonds are sigma bonds (head-to-head overlap of orbitals) with one point of overlap so can rotate

Question 2

Describe the bonding in alkenes

Answer 2

In alkenes:

• C only bonds to 3 other atoms
• The orientation is planar & these angles to 120 to minimise repulsion
• The second C-C bond is a π-bond, formed by p-orbitals overlapping side-to-side w/ 2 points of overlap

Question 3

Give two consequences of π-bonding

Answer 3

• Geometric isomerism: two points of overlap prevents the C=C bond from rotating
• Reactivity: the electrons in this π-bond stick out above & below the molecule so they are more accessible for reactions

Question 4

Do alkanes & alkenes display geometric isomerism?

Answer 4

Alkanes: don’t as C-C bond can rotate
Alkenes: do since C=C double bond doesn’t usually rotate

Question 5

Give 2 key features of geometric isomers

Answer 5

• Non-superimposable forms of the same alkene

- There must be 2 diff groups on both ends of the double bond

Question 6

How can you check if a molecule displays geometric isomerism?

Answer 6

Split the alkene into two by drawing a line through the middle perpendicular to the double bond

Question 7

Give the two ways of naming geometric isomers

Answer 7

• E/Z notation (IUPAC preferred method)

- Cis/Trans notation

Question 8

How are the groups arranged in ‘cis’ isomers?

Answer 8

Same groups/atoms are on the SAME SIDE of the double bond

Question 9

How are the groups arranged in ‘trans’ isomers?

Answer 9

Same groups/atoms on the OPPOSITE SIDES of the double bond

Question 10

How are the groups arranged in ‘Z’ isomers?

Answer 10

Same groups/atoms are on the SAME SIDE of the double bond

Question 11

How are the groups arranged in ‘E’ isomers?

Answer 11

Same groups/atoms on the OPPOSITE SIDES of the double bond

Question 12

How does the use of E/Z notation differ from cis/trans notation?

Answer 12

E/Z isomerism - when there are only TWO different groups E/Z isomerism works the same way as cis-trans

Question 13

What do we have to think about when there are more than two groups in E/Z isomerism?

Answer 13

Priority

Question 14

How do you work out the priority of groups?

Answer 14

1. First split the molecule down the middle, perpendicular to the double bond to divide into the two ends
2. Compare the atomic numbers of atoms connected to each C
3. Now split it the other way
4. If both the higher priority groups are on the same side it is a Z isomer