Haloalkanes

Question 1

What is

a primary haloalkane?

Answer 1

This is a haloalkane that has 1 alkyl group attached to the carbon in the C-X bond.
It can be represented as: RCH₂X

Question 2

What is

a secondary haloalkane?

Answer 2

This is a haloalkane that has 2 alkyl groups attached to the carbon in the C-X bond.
It can be represented as: RR’CHX

Question 3

What is

a tertiary haloalkane?

Answer 3

This is a haloalkane that has 3 alkyl groups attached to the carbon in the C-X bond.
It can be represented as: RR’R”CX

Question 4

What makes nucleophilic substitution of haloalkanes possible?

Answer 4

The polar nature of the C-X bond

Where ‘X’ is the halogen atom

This gives the carbon a δ+ charge, allowing it act as an electrophile

Question 5

What factor determines the reactivity of haloalkanes?

in nucleophilic substitution

Answer 5

The bond energy of the C-X bond

Question 6

What type of haloalkanes can we expect to be the most reactive?

Answer 6

Iodoalkanes

The C-I bond has the lowest bond energy, hence it is broken most easily

Question 7

What else is produced when haloalkanes are the hydrolysed to alcohols?

Answer 7

The sodium/potassium salt of the corresponding halide
R-X_(aq) + OH^-_(aq) → R-OH_(aq) + NaBr_(aq)

Question 8

What are the reaction conditions for the hydrolysis of haloalkanes to alcohols?

Answer 8

1. A mainly aqueous medium
2. NaOH / KOH
3. Refluxing of the system
4. (light) Heating

Question 9

What trend can we identify in the C-X bonds in haloalkanes?

Answer 9

The strength of the decreases down the group

The C-F bond has the highest bond energy and is one of the strongest single bonds in Chemistry

Question 10

What is the result when an ethanolic medium is used for the hydrolysis of haloalkanes?

Answer 10

An elimination reaction occurs and an alkene is formed instead

with water (the small molecule eliminated) and the halide salt as byproducts

Question 11

What role is played by NaOH when reacting with a haloalkane in an ethanolic medium?

Answer 11

It acts as a base

It accepts H⁺ (gotten from the haloalkane) to form water. Since two atoms are removed from one carbon in the haloalkane, it forms a double bond with and adjacent carbon

Question 12

What are the reaction conditions for the hydrolysis of haloalkanes to alkenes?

Answer 12

1. An anhydrous medium
2. NaOH / KOH in ethanol
3. Refluxing of the system
4. (light) Heating

Question 13

What is the name of the reaction mechanism

for the reaction of water or OH^- and a haloalkane?

Answer 13

Nucleophilic substitution

Question 14

What is the name given

for the nucleophilic substitution of a haloalkane with water?

Answer 14

Hydration

Question 15

What is the name given

for the nucleophilic substitution of a haloalkane with OH^-?

Answer 15

Hydrolysis

Question 16

Which is a better nucleophile and why :

water or OH^-?

Answer 16

OH^-is a better nucleophile than water. the Oxygen atom in water acts as the nucleophile and it has a partial negative charge while in OH^-, the oxygen atom has a fully negative charge hence is more strongly attracted to a region of e^- density

Question 17

Which should occur faster

hydration or hydrolysis of haloalkanes?

Answer 17

Hydrolysis

Question 18

How many reaction mechanisms are possible

For the hydrolysis of haloalkanes?

Answer 18

Two. S_N1 and S_N2 reaction mechanisms

Question 19

What is the difference between

S_N1 and S_N2 reaction mechanisms?

Answer 19

In the S_N1 reaction mechanism, the C-X bond is broken first before the C-OH bond is formed.
In the S_N2 reaction mechanism, the breaking of the C-X bond and the formation of the C-OH bond happen simultaneously

Question 20

How does the nature of the haloalkane (primary, secondary or tertiary)

affect the reaction mechanism it takes?

Answer 20

Primary & Secondary haloalkanes undergo the S_N2 reaction mechanism while Secondary & Tertiary haloalkanes undergo the S_N1 reaction mechanism

Question 21

what is the ‘meaning’ of

S_N1?

Answer 21

• S = “Substitution”
• N = “Nucleophilic”
• 1 = “(rate) 1”

This means there is only one concentration (conc. of the haloalkane) determining the rate of the reaction in the slow step

Question 22

what is the ‘meaning’ of

S_N2?

Answer 22

• S = “Substitution”
• N = “Nucleophilic”
• 1 = “(rate) 2”

This means there are two concentrations (conc. haloalkane & conc. hydroxide) determining the rate of the reaction in the slow step

Question 23

In which reaction mechanism

is the nucleophile involved from the onset?

Answer 23

The S_N2 mechanism

In S_N1, the bond is broken first before the nucleophile is introduced

Question 24

What is the slow step in a chemical reaction?

Answer 24

This is the slowest step in a reaction which determines the rate at which it occurs

Question 25

In the hydration of a haloalkane, an intermediate is formed **with a *positive charge*** on the Oxygen atom. What is the matter?

Answer 25

The oxygen atom is sharing its valence e^- in so many regions. It still has the 2 O-H bonds from the water molecule and the **new bond it has formed** with carbon

Question 26

How does oxygen rectify the positive charge it has in the next step?

Answer 26

It withdraws bonding e^- from one of the hydrogen atoms, the bond is split **heterolytically**, making a H⁺ ion

Question 27

What are the products of the hydration of a haloalkane?

Answer 27

an alcohol and a **hydrogen halide** R-X_(aq) + H₂O_(l) → R-OH_(aq) + HBr_(aq)