Reactivity 3.4 HL

Question 1

Lewis acid

Answer 1

Can accept a pair of electrons

Question 2

Bronstead-Lowry base

Answer 2

Accepts a proton

Question 2

Lewis base

Answer 2

Can donate a pair of electrons. Must have at least one pair of non-bonding electrons. All ligands are Lewis bases.

Question 3

Bronstead-Lowry acid

Answer 3

Donates a proton

Question 4

Difference between Bronstead-Lowry acid/base and Lewis acid/base

Answer 4

Bronstead-Lowry are defined in terms of donating or accepting protons, Lewis is defined in terms of a trasnfer of electrons (ie acts as a BL base by accepting a proton and lewis base as it donates a pair of electrons to a proton)

Question 5

Why are Lewis acids electron deficient?

Answer 5

They are all able to add at least one extra pair of electrons to its outer shell.

Question 6

How is a coordination bond formed with Lewis bases?

Answer 6

A Lewis base donates a pair of electrons to a Lewis acid. Both of the electrons originate from the same atom, so it is a coordination bond.

Question 7

Halogenoalkanes react with …. to form….

Answer 7

Warm dilute sodium hydroxide to form alcohols

Question 8

Rate of reaction for primary halogenoalkanes

Answer 8

Depends on the concentration of both the halogenoalkanes and the hydroxide ion

Question 9

Rate of reaction for tertiary halogenoalkanes

Answer 9

Depends only on the concentration of halogenoalkanes and is independent on the concentration of hydroxide ions.

Question 10

Sn

Answer 10

Substitution nucelophilic

Question 11

Sn2

Answer 11

Substitution nucleophilic bimolecular

Question 12

Rate determining step for the reaction of primary halogenoalkanes with hydroxide ions

Answer 12

Both the halogenoalkane and the nucleophile

Question 13

Unstable five-member transition state

Answer 13

Where the carbon is weakly bonded to both the halogen and the nucleophile

Question 14

Sn1

Answer 14

Substitution nucleophilic monomolecular

Question 15

Two steps in a Sn1 reaction

Answer 15

Step 1 (slow) - heterolytic breaking of the C-halogen bond. Produces a halide ion and carbocation
Step 2 (fast) - reaction between nucelophile and carbocation

Question 16

Primary halogenoalkanes mechanism

Answer 16

Sn2 due to steric reasons (enough space for nucleophile to approach)

Question 17

Tertiary halogenoalkanes mechanism

Answer 17

Sn1 due to the inductive effect of the R-groups. Tertiary carbocations are the most stable.

Question 18

Carbocation stability

Answer 18

Primary (least), secondary, tertiary (most)

Question 19

Secondary halogenoalkanes mechanism

Answer 19

React with Sn1 and Sn2

Question 20

Sn2 summary

Answer 20

Primary halogenoalkanes, concerted one step mechanism with unstable transition state. Second order, bimolecular.

Question 21

Sn1 summary

Answer 21

Tertiary halogenoalkanes, two step reaction via carbocation intermediate and two unstable transition states. First order, unimolecular

Question 22

What are the three factors that affect the rate of nucleophilic substitution?

Answer 22

Type of halogenoalkane, nature of the halogen (leaving group) and the nature of the nucleophile

Question 23

How does the type of halogenoalkane affect the rate of nucleophilic substitution?

Answer 23

• Tertiary > secondary > primary
• They go via an ionic mechanism

Question 24

How does the nature of the halogen affect the rate of nucleophilic substitution?

Answer 24

- Iodo>bromo>chloro - C-I is the weakest bond. This means that it is a better leaving group as less energy is required to break the halogen bond heterolytically.

Question 25

How does the nature of the nucleophile affect the rate of nucleophilic substitution?

Answer 25

The more electron dense the nucleophile, the faster the reaction.

Question 26

Why is OH- a better nucleophile than water?

Answer 26

It has a greater negative charge density and is attracted more strongly to an electron deficient carbon atom/ion.

Question 27

Ammonia reacting to for amines (non-bonding electrons, nucleophiles)

Answer 27

Amines still contain a nonbonding pair of electrons on the nitrogen and so behave as a nucleophile and can form secondary, tertiary and quaternary amines.