Mixture of Isomerism Questions
Carbonyls and Optical Isomerism Worksheet
1. E-Z Isomerism
Which of the following molecules exhibits E-Z isomerism?
a) CF₂=CF₂
No. The groups on each carbon atom are identical (Two Fluorines on each side).
b) CH₃CH=CHCH₃
Yes. Each carbon in the double bond is attached to two different groups (a Hydrogen and a Methyl group).
c) (CH₃)₂C=CHCH₃
No. The first carbon atom is attached to two identical methyl groups.
d) CBr₂=CBr₂
No. The groups on each carbon are identical (Two Bromines).
2. Hydration of But-2-ene
But-2-ene can react with water to produce an alcohol.
a) State the conditions for the reaction.
Conditions: Phosphoric acid catalyst, water as steam (gas).
b) Draw the display formula for the product.
c) Draw the geometry for the two isomers formed in this reaction.
3. Reaction of But-1-ene
But-1-ene reacts with hydrogen chloride.
a) State the name of the mechanism of the reaction.
Electrophilic Addition
b) The reaction can produce three different isomers. Explain how the reaction can form three different isomers. Include the mechanisms in your explanation, and explain each step in the mechanism. Additionally, include an explanation for which isomer is the most common.
Explanation of the mechanism:
- There is a dipole on the HCl molecule, resulting in the hydrogen being partially positive.
- The pi bond electrons in the C=C are attracted to the partially positive hydrogen, and they form a bond with it.
- The bond between the hydrogen and the chlorine breaks heterolytically because of the polar nature of the bond, with the electron pair moving onto the chlorine, producing a chloride ion.
- These interactions produce a carbocation on either carbon one or carbon two. The chloride ion is attracted to the positive carbocation and forms a bond with it.
Explanation of the three isomers:
- The two structural isomers formed are 1-chlorobutane (from the primary carbocation) and 2-chlorobutane (from the secondary carbocation).
- 2-chlorobutane exists as two optical isomers (enantiomers) because the secondary carbocation is trigonal planar, allowing the chloride ion to attack from above or below the plane with equal probability.
Explanation for the most common product:
- The two possible carbocations have different stabilities.
- The secondary carbocation is more stable than the primary carbocation because it has more alkyl groups attached to it. As alkyl groups have a positive inductive effect, they stabilise the cation, resulting in a more stable intermediate.
- This means that structurally, 2-chlorobutane is formed in a greater amount than 1-chlorobutane.
- However, because 2-chlorobutane splits into two enantiomers (racemic mixture), 1-chlorobutane is statistically the most common single isomer out of the three.
4. Nucleophilic Addition & Optical Isomerism
Chloroethanal reacts with potassium cyanide followed by a dilute acid.
a) Draw the two isomers that are formed in the reaction and show how they relate to one another.
b) Describe how the two different isomers could be distinguished from one another.
The two isomers can be distinguished by shining plane polarised light through samples of each isomer. One isomer will rotate the plane of the light clockwise and the other will rotate the plane of the light anticlockwise by the same amount.