4.5 Haloalkanes

Question 1

What are haloalkanes ?

Answer 1

Haloalkanes are compounds containing carbon, hydrogen and at least 1 halogen.

Question 2

What are aliphatic haloalkanes ?

Answer 2

Halogen joined to a straight chain or branched carbon chain.

Question 3

What is a primary haloalkane ?

Answer 3

0/1 alkyl groups attached to carbon with halogen.

Question 4

What is a secondary haloalkane ?

Answer 4

2 alkyl groups attached to the carbon atom with halogen attached.

Question 5

What is a tertiary haloalkane ?

Answer 5

3 alkyl groups attached to carbon atom with halogen attached.

Question 6

What type of bond is the halogen-carbon bond in a haloalkane ?

Answer 6

Polar bond are halogens are more electronegative than carbon so pair of electrons in the bond are closer to the halogen than the carbon.

Question 7

Why can carbon atom attract a nucleophile in haloalkane ?

Answer 7

Carbon has slight positive charge so attracts nucleophile, that acts as an electron pair donor.
Eg., OH-, H20, NH3

Question 8

What is a nucleophile ?

Answer 8

A chemical species that can donate a pair of electrons to a different chemical species (generally to an electrophile) to form a chemical bond in a reaction.

Question 9

What is nucleophilic substitution of haloalkanes ?

Answer 9

When primary haloalkanes react with a nucleophile in a substitution reaction, where the halogen is substituted for the nucleophile.

Question 10

What is hydrolysis of haloalkanes ?

Answer 10

Chemical reaction involving water or an aqueous solution that causes the braking of a bond in a molecule.

Question 11

What is the first step in nucleophilic substitution ?

Answer 11

Nucleophile approaches carbon atom attached to the halogen from the opposite side of the molecule.

Question 12

What is minimised by the direction of attack in nucleophilic substitution ?

Answer 12

Minimises repulsion between nucleophile and delta negative halogen.

Question 13

What is attracted to carbon atom in nucleophilic substitution ?

Answer 13

Lone pair of electrons is attracted and donated to the delta positive carbon atom attached to the halogen.

Question 14

What occurs when attraction is formed between lone pair of electrons and carbon atom of haloalkane ?

Answer 14

-Carbon-halogen bond is broken by heterolytic fission.
-New bond is formed between oxygen, from nucleophile, and carbon atom on haloalkane.
This forms new organic product, an alcohol, and a halide ion.

Question 15

What is the rate of hydrolysis determined by in a haloalkane ?

Answer 15

Depends on strength of carbon-halogen bond in haloalkane.
C-F bond is strongest
C-I bond is weakest

Question 16

Why is rate of hydrolysis determined by bond strength in haloalkane ?

Answer 16

Less energy is requires to break weaker bonds so rate of hydrolysis is quicker.

Question 17

How do you measure rate of hydrolysis ?

Answer 17

The rate can be followed by presence of aqueous silver nitrate. As reaction takes place, halide ions are produced, which react with Ag+ to form a precipitate of a silver halide.

Question 18

Why are haloalkanes insoluble in water ?

Answer 18

They are unable to form hydrogen bonds with water, despite have polar bonds. Therefore, an ethanol solent is used in hydrolysis as it allows haloalkane to dissolve and mix with water to produce a precipitate.

Question 19

What is the property of boiling points of haloalkanes compared to their alkane counterparts ?

Answer 19

Higher boiling points due to increased London forces.

Question 20

What is the flammability of haloalkanes compared to their alkane counterparts ?

Answer 20

Less flammable due to fewer C-H bonds.

Question 21

Where are haloalkanes used ?

Answer 21

Used in electronic industry as a flame retardant (BFR’s).
CFC’s were used in refrigerators and air conditioning units, but now HFC’s are used for refrigerators.

Question 22

Where is the ozone layer ?

Answer 22

At the edge of the stratosphere, which is above the troposphere (lowest part of Earth’s atmosphere).

Question 23

What is stratospheric ozone ?

Answer 23

‘Good’ ozone as in this level, it acts to block UV radiation from the sun.

Question 24

What is tropospheric ozone ?

Answer 24

‘Bad’ ozone as this acts as a harmful pollutant to humans, animals and the environment due to it being close to Earth’s surface.

Question 25

Why are CFC’s stable ?

Answer 25

Stable due to carbon-halogen bond strength

Question 26

When do CFC’s remain stable until ?

Answer 26

Remain stable until they have reached the stratosphere, where they begin to break down, which forms chlorine radicals, which then react with ozone catalysing ozone depletion.

Question 27

Why is UV radiation bad when CFC’s reach the stratosphere ?

Answer 27

UV radiation provides enough energy to break the carbon-halogen bond by homolytic fission, so carbon-chlorine bond is broken.

Question 28

What is another ozone depletion reaction?

Answer 28

Nitrogen oxide also catalyses the breakdown of ozone as nitrogen oxide radicals are formed naturally during lightening strikes.