AS Chemistry - Unit 2

have  an alkane skeleton with 1 or more halogen atoms in place of a hydrogen atom.

General Formula - CnH2n+1

Naming -                    CH3CH2CH2Cl = 1 - chloropropane

F - Fluoro                   if theres more than 1 halogen:

Cl - Chloro                 2 = di

Br - Bromo                 3 = tri

I - Iodo                       4 = tetra

example: CH2BrCHBrCH2CH3      1, 2, di-bromobutane

Q1.) Name: CH3CH2Cl

Q2.) Name: CH3CHFCHICH2I

Q3.) What is the general formula for a haloalkane?

Q4.) Describe what a haloalkane is?

Q5.) Name: CH3CHFCHCl2

Q6.) Name: CH3CHFCH3

Q7.) Name: CH3CF2CFHCH2CH3

Q8.) Name: CH3CH2CHBrCH3

Bond Polarity: Haloalkane bond is polar - the halogen is more electronegative than the carbon atom.

Fluorine - most electronegative --> Iodine least (decreases down group)

Solubility, no - the polar bond isn't polar enough to make soluble in H2O

Boiling Point - depends on the no. of carbon atoms & halogen atoms

increases with chain length

Fluorine lower b.P ------> Iodine highest b.P - increases down group - larger molecules = stronger Van Der Waals' forces.

Haloalkanes higher b.P than alkanes - higer relative mass & more polar

2 Factors - bond polarity & bond enthalpy

Enthalpy: gets weaker going down the group   

F - Strongest - small atom - e- pair strongly attracted (closer) = strong bond

I - weakest - large atom - e- pair futher distance from nucleus = weaker attraction

Polarity:

Halogens more electronegative than carbon - can be attracted by reagents that are electron rich areas - nucleophiles

(bond enthalpy more important than polarity)

Making a Alcohol:

Reaction: Nucleophilic Subsitution

Conditions: warm & aqueous

Reagant: NaOH

Yield: low due to other unwanted reactions also taking place

Product: an alcohol

Draw Mechanism:

Making a Nitrile:

Reaction: Nucleophilic Subsitution

Conditions: warm & alcoholic (ethanol)

Reagant: KCN

Product: Nitrile

Easily converted into carboxylic acid, increases C-C chain (C from alcohol)

Draw Mechanism:

Making a Amine (Primary)

Reaction: Nucleophilic Subsitution

Conditions: Increased Pressure

Product: Amine   NH3

Yield: low several  different products are possible

Draw Mechanism:

Making a Alkene

Reaction: Elimination

Conditions: Alcoholic (ethanol) insteed of H2O

Reagant: NaOH or KOH   (OH acts as a base for the reaction)

If the halogen atom is in a central position then there will be 2 possible products.

CH3CH2CHBrCH3      = a mixture of but-1-ene & but-2-ene

(but-2-ene the major product)

(but-1-ene the minor product)

Making a Haloalkane

Reactants - Alkanes + Haloalkane

Conditions: UV light

Mechanism: Free Radical Subsitution

Free Radical - species with an unpaired electron, which makes them very reactive.

3 Step Mechanism

1. Initiation

2. Propogation (chain reaction)

3. Termination (2 free radicals react = a stable compound, slows down reaction)

This reaction is a v.poor way to make haloalkanes because it produces a number of products.

Chlorine is normally used but bromine can also be used:

Draw Bromine Mechanism:

Chemicals called CFC's (chloro, fluoro compounds) were used in fridges and spray cans. CFC's excaped into the atmosphere the sunlight (UV) decomposed them to give chlorine radicals.

This radicals reacted with the Ozone molecules in a chain reaction.

1 chlorine radical could distory thousands of O(3) molecules.

Reduction in O(3) caused increase in diseases e.g. skin cancer, catarast.

so, CFC's were banded & replaced with less harmful substances MEPB

Q1.) The haloalkanes become more reactive in nucleophilic substitution         1-chlorobutane < 1-bromobutane < 1-iodobutane explain why?

Q2.) What type of compund is formed when 1-bromobutane reacts with:

• aqueous OH:-
• alcoholic CN-
• conocentrated NH3

Q3.) Draw the mechanism for each?

Q4.) Explain what is meant by a nucleophile

Q5.) Explain what is meant by free radical?

Q6.) Write an equation for the initiation step use Cl then Br

Q7.) Write two equations for the propagating step, using methane and chlorine as reactants?

Q8.) Write an equation for 2 possible terminating steps?

Q9.) Explain why the reaction is very inefficient?

Q10.) Write a mechanism for making a alcohol, state the conditions?

Q11.) Comment on the yield of this reaction?

Q12.) What is the name of the reaction which products an alkene?

Q13.) What other product is formed in this reaction?

Q14.) Why is but-2-ene described as the major product?