Halogenalkanes

Question 1

Physical properties?

Answer 1

• halogen has a higher electronegativity than the carbon it’s attactcjed to.
• the presence of polar bond and the slight charges result in increased IMF between the molecules. It’s the PDD force that needs to be overcome in order to melt/boil a sample. Therefore an increase in BP/MP and density in comparison to corresponding alkane
• don’t posses H2 bond so they are insoluble in water but they are more soluble in water than corresponding alkane/alkene

Question 2

Nucleophillic Substitution reactions

Answer 2

• these occur when a molecule is attacked by a nucleophile and an atom/group of atoms is replaced by nucleophile. Nucleophiles are attracted towards a slightly + charge.

Alkali hydrolysis of haloalkane
• Reagents : aq NaOH or KOH
• conditions : heat under reflux
• halogenalkane –> alcohol

General mechanism for nucleophillic
- the nucleophile approaches the + charged carbon atom on opposite side to halogen. This way the repulsion between nucleophile and halogen is minimum. As a bond firms between nucleophile/carbon atom holding halogen, the carbon-halogen bond is broken

Question 3

Bond enthalpies?

Answer 3

• as you go from C-F to C-I the bond enthalpies decrease, the bond gets weaker, it becomes easier to break the carbon to halogen bond therefore it’s easier to loose the halide: the halogenalkane becomes more reactive
• hydrolysis of the halogenalkane occurs most rapidly with iodoalkanes. Can be observed by aq ethanolic silver nitrate and timing the first appearance of a sliver halide precipitate. The halogen ions liberated will react w/ sliver ions in solution to form precipitate.

Question 4

Uses of halogenalkanes?

Answer 4

• PVC ( polychlroethene )
- used in plastic carrier bags , to make insulation on electrical wires, window. It’s waterproof and doesn’t react w/acids

PTFE ( polytetrafluroethene ) is resistant to hear and chemical corrosion. It’s found on non stick cookware.

• CFC’s
- use as aerosols, refrigerants and in air conditioning due to low volatility reactivity and non toxicity

Question 5

Environmental consideration?

Answer 5

• CFC’s are blamed for causing damage to the environment. They absorb more infra red than CO2, their contribution to the G.H.E is very low due to their low abundance in the atmosphere.
• CFC’s are responsible for thinning of the ozone layer in stratosphere. When they reach the stratosphere CFC’s absorbs UV which causes photodissociation of carbon-chlorine bonds
• very reactive chlorine radicals are formed. These radicals catalyse the decomposition of the ozone to O2.

Question 6

What are scientists doing?

Answer 6

• working new materials, which causes less damage to the environment the 1,1,1,2-tetrafluroethane is now used as an alternative to CFCs, the presence of the H2 atom increases reactivity if the compound and it undergoes reaction in the lower atomosphrre so does not produce damaging free radicals
• CO2 now used as a blowing agent for producing expanded polymers instead of CFc’s

Question 7

Why are halogenalkanes reactive?

Answer 7

• the carbon-halogen bond is polar giving rise to its reactivity
• the slightly positive carbon doesn’t have enough electrons and so it’s attacked by nucleophiles
- nucleophiles: an electron pair donor
It donates electrons to somewhere without enough electrons

Question 8

Bond enthalpies?

Answer 8

• as we go from C-F to C-I:
: the bond enthalpies decrease as the bonds get weaker - it becomes easier to break the carbon to halogen bond and so easier to lose the halide ion. The halogenalkanes get more reactive