11.3 Alkenes, Set A Answer Key To print: Ctrl + P, paper A4, margins Default.
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11.3

Alkenes

Set AAnswer key
A

From the lesson

10 marks
1

Only (b). In (a), C1 has two H. In (c), C2 has two CH3.[1]

2

2-bromobutane, CH3CHBrCH2CH3[1]

H adds to C1 (more H atoms), giving a 2° carbocation[1]

stabilised by two electron-donating alkyl groups which disperse the positive charge more than in the 1° carbocation.[1]

3

Propanone comes from the =C(CH3)2 end of the alkene, and ethanoic acid comes from the =CHCH3 end.[1]

(CH3)2C=CHCH3, 2-methylbut-2-ene[1]

4

Restricted rotation about C=C, and each C of C=C has two different groups.[1]

5

Steam, H3PO4 catalyst, 300 °C, 60 atm.[1]

6

Electrophilic addition.[1]

7

CO2 and H2O.[1]

B

Practice

19 marks
8

CIt is an electrophile.[1]

It is an electrophile, as the carbon carrying the positive charge is electron deficient.

AOnly the carbon carrying the positive charge in the intermediate is trigonal planar. The other carbon has a tetrahedral geometry. BThe carbocation intermediate has a positive charge. DThe intermediate does not have any unpaired electrons.
9

BNeither reacts with bromine to give 1,4-dibromobutane.[1]

The electrophilic addition of bromine to an alkene occurs across adjacent carbon atoms. CH3CH2CH=CH2 gives CH3CH2CHBrCH2Br (1,2-dibromobutane), while CH3CH=CHCH3 gives CH3CHBrCHBrCH3 (2,3-dibromobutane).

ACH3CH=CHCH3, but not CH3CH2CH=CH2, can exhibit cis-trans isomerism. CBoth react with hydrogen to form butane. DBoth may be oxidised by alkaline potassium manganate(VII) to form a diol, due to the presence of C=C.
10

A C=C double bond consists of a sigma (σ) bond and a pi (π) bond. The π bond is a region of high electron density and is susceptible to attack by electrophiles (electron deficient species).[1]

Many reactions involve breaking the weak π bond while the σ bond remains intact. Alkanes are less reactive, as they contain the stronger σ bonds only.[1]

11
(a)

Reagent: Br2 in CCl4. Condition: dark, room temperature. Bubble a sample of each gas through Br2 dissolved in CCl4.[1]

When ethene is bubbled through, orange Br2 is decolourised: CH2=CH2 + Br2 → CH2BrCH2Br. Orange Br2 does not decolourise when ethane is bubbled through it.[1]

(b)

Reagent: KMnO4(aq), H2SO4(aq). Condition: heat. Bubble a sample of each gas into a mixture of several drops of KMnO4(aq) in excess sulfuric acid, and heat.[1]

With but-1-ene, the purple KMnO4 is decolourised and a colourless gas (CO2), which forms a white precipitate when bubbled through Ca(OH)2(aq), is evolved. CH3CH2CH=CH2 + 5[O] → CH3CH2COOH + CO2 + H2O,   CO2 + Ca(OH)2 → CaCO3 + H2O[1]

With but-2-ene, the purple KMnO4 is decolourised, but no gas is evolved. CH3CH=CHCH3 + 4[O] → 2CH3COOH[1]

12

Mechanism: electrophilic addition

[3]

13
(a)

A: cis-but-2-ene   B: trans-but-2-ene [1]

C: but-1-ene, CH2=CHCH2CH3   D: 2-methylpropene, (CH3)2C=CH2[1]

E: cyclobutane   F: methylcyclopropane [1]

(b)

A is polar (non-zero net dipole moment), while B is non-polar (zero net dipole moment).[1]

The intermolecular forces between molecules of cis-but-2-ene (permanent dipole–permanent dipole and instantaneous dipole–induced dipole interactions) are stronger than those between molecules of trans-but-2-ene (only instantaneous dipole–induced dipole interactions). Since boiling involves overcoming the intermolecular forces, cis-but-2-ene has a higher boiling point than trans-but-2-ene.[1]

(c)

A: CH3COO−Na+   B: CH3COO−Na+[1]

C: CH3CH2COO−Na+ + Na2CO3   D: CH3COCH3 + Na2CO3[1]