Haloalkanes and Haloarenes

Question 1

What is the ease of replacement of hydrogen atoms from an alkane?

Answer 1

3>2>1

Question 2

Halogenation takes place via what mechanism?

Answer 2

Free radical mechanism

Question 3

The reactivity of halogens follow which order?

Answer 3

F2>Cl2>Br2>I2

Question 4

The addition of halogen acids to alkanes takes place via ………………………….

Answer 4

electrophilic addition

Question 5

The addition of halogen acids to alkanes takes place via electrophilic addition except for in the case of ………………. where in it takes place via …………………..

Answer 5

• HBr in the presence of organic peroxides

* Free radical mechanism

Question 6

Hundsdieker’s rxn is predominantly used for which halide?

Answer 6

Br2 , it produces very low yield for I2 and Cl2

Question 7

What are the physical states of the different haloalkanes?

Answer 7

The lower ones are gaseous and the higher ones are solid.

Question 8

What are beta elimination rxns used for?

Answer 8

Preparation of alkenes

Question 9

What is Saytzeff’s rule?

Answer 9

The most substituted alkene will be the major product.

Question 10

What is the order of the ease of dehydrohalogenation? Why?

Answer 10

3 > 2 > 1
This can be explained using Saytzeff’s rule. This rule states that the more substituted alkane is more common. This means that its formation is faster and it is more stable. This means that an alkane must undergo faster reaction to form its compound. Hence a tertiary alkane undergoes faster rxn and is hence more reactive.

Question 11

Why is dry ether used in the preparation of Grignard’s agents?

Answer 11

This is because they get decomposed in even trace amounts of moisture

Question 12

Talk about the melting points and boiling points of Haloalkanes.

Answer 12

The are much higher than those of the parent alkanes.
This is because haloalkanes have a higher molecular mass and hence have stronger van der waals forces. The existence of slight dipole dipole interactions also enhances their mp and bp.

Question 13

What is the trend observed w.r.t to the mp and bp of haloalkanes.

Answer 13

• bp increases with increase in mass of halogen
• bp increases with more substituted groups
• bp decreases with branching (decreases the surface area and hence they cannot interact with all the molecules around.)

Question 14

Talk about the solubility of haloalkanes.

Answer 14

They are insoluble in water. But they are soluble in organic solvents.

Question 15

Why is chloroform no longer used as an anaesthetic?

Answer 15

This is because it is toxic.

Question 16

In what ways can chloroform be stored in an effective way?

Answer 16

• Dark bottles
• The bottles should be filled
• A small quantity of ethanol should be added to chloroform .

Question 17

Which reactant is used more for the preparation of chloroform?

Answer 17

Acetone

Question 18

The lewis acids used in this rxn is also called ….?

Answer 18

Halogen carrier

Question 19

What is directive influence?

Answer 19

This says that a group already present on the aromatic ring decides the position of the incoming group.

Question 20

Name some ortho and para directing groups.

Answer 20

NHR , NR2 , OH, OR, R, Cl, Br, I

Question 21

Name some meta directing groups.

Answer 21

NO2, CN, SO3H, CHO, COOH, CONH2

Question 22

What is the physical state of haloarenes?

Answer 22

They are colourless , crystalline solids

Question 23

Explain the solubility if Haloarenes

Answer 23

They are insoluble in water due to low polarity.

Question 24

Explain the trends in the mp and bp of haloarenes

Answer 24

• The mp and bp of haloarenes increases with the size of halogen atom.
• The bp of isomeric haloarenes doesn’t vary to a great degree.
• The mp of isomeric haloarenes varies greatly.
• The mp of para isomers is the greatest (symmetry and high intermolecular forces)

Question 25

Compare the nucleophilic substitution rates of haloalkanes and haloarenes.

Answer 25

https://www.youtube.com/watch?v=GEjSinZ7cpQ

Haloalkanes react faster than haloarenes due to formation C=Cl double bond on haloarene.