Mechanism Worksheet 1: SN1 and SN2
1. Halogenoalkanes undergo nucleophilic substitution reactions. The rates and mechanisms of these reactions
depend on whether the halogenoalkane is primary, secondary or tertiary. Explain the term nucleophilic
substitution.
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(2)
2. The formula C4H9Br represents more than one compound. Using this formula, draw a structure (showing all bonds
between carbon atoms) to represent a halogenoalkane that is
(i) primary.
(1)
(ii) secondary.
(1)
(iii) tertiary.
(1)
(Total 5 marks)
3. The reaction of aqueous KOH with 1 bromobutane occurs by a SN2 mechanism.
(a) Draw the mechanism for this reaction, including the structural formulas of 1-bromobutane, the
transition state and the organic product.
(b) State and explain how the above reaction is affected when the concentration of the KOH is doubled.
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(c) State and explain how the rate of reaction of 1-chorobutane in the above SN2 reaction compares with that
of 1 bromobutane.
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4. Bromoethane reacts with ammonia as follows.
CH3CH2Br + NH3 → CH3CH2NH
CH3CH2NH
+ Br−
+ NH3 → CH3CH2NH2 + NH
The mechanism for this reaction is described as SN2.
(a) State the meaning of each of the symbols in SN2.
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(2)
(b) State the name of the organic product of the reaction, CH3CH2NH2.
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(1)
(c) Explain, using “curly arrows” to show the movement of electron pairs, the mechanism of the attack by
ammoniaon bromoethane, and show the structure of the transition state.
Answer key for Worksheet
1 change / replacement of atom / group (in molecule);by species with a non-bonding / lone pair of electrons / attracted to electron
deficient part of molecule (OWTTE) / Lewis base; 2
2.
1
(ii) CH3–––CH2–––CHBr–––CH3 1
(iii)
1
Position of Br must be clearly shown
In (i), (ii) and (iii), all C—C bonds must be shown. Do not penalize missing Hatoms.
2
(a)
suitable diagram with structure of 1-bromobutane;
two correctly positioned curly arrows;
(second one must start from O or – sign)
transition state structure with partial bonds to OH and Br and a
negative charge;
Correct structure of butan-1-ol; 4
(b) the rate of the reaction doubles;
the rate is proportional to [OH–] / OH− appears in the rate-determining
step / first order with respect to OH−; 2
[8]
3 substitution;
nucleophilic;
bimolecular / two species in rate-determining or slowest step;
Do not accept second order.
Three correct [2] , two correct [1] , one correct [0].
2 max
(b) aminoethane / ethylamine; 1
(c) curly arrow from N of NH3 to C joined to Br;
curly arrow from C—Br bond to Br;
central C of transition state joined to CH3, H, H, Br and NH3;
transition state with no charge and with --- bonds to NH3 and Br; 4
Ignore products.
[7]