L12 - halogenoAlkanes

objectives

• Recap structural features and naming of haloalkanes

Structure of Haloalkanes

Contain the functional group C-X where X is a Group VII element (halogen (F,Cl,Br or I))

Names: Based on original alkane with a prefix indicating halogens and position.

CH₃CH₂CH₂Cl 1-chloropropane CH₃CHClCH₃ 2-chloropropane

CH₂ClCHClCH₃ 1,2-dichloropropane

CH₃CBr(CH₃)CH₃ 2-bromo-2-methylpropane

Position Isomers in Haloalkanes

Different structures are possible due to Different positions for the halogen and branching of the carbon chain

When a functional group can be on more than one carbon we see that this has the same molecular formula but a different structural formula - in chemistry where this happens we call this Isomerism

Position Isomerism is when the same functional group that can be placed on different carbons in the carbon chain.

Properties of Haloalkanes

As Atoms increase in size as you down a group in the periodic table the size of the intermolecular forces between the molecules also increases.

Therefore the bigger the halogen atom, the higher the boiling point

As all atoms are greater in size than Hydrogen when a Hydrogen atom is replaced by another Halogen the size of the intermolecular forces also increases so..

The more halogen atoms there are, the higher the boiling point

Primary, Secondary and Tertiary Haloalkanes

When a Halogen is bonded to a Carbon chain the carbon it is attached to can affect the chemistry of the compound.

Primary Haloalkane:

• The Halogen is bonded to a carbon that is only bonded to one other carbon
• These are usually found at the end of a carbon chain

Secondary Haloalkane:

• The Halogen is bonded to a carbon that is bonded to two other carbons
• These are usually found in the middle of a carbon chain

Tertiary Haloalkane:

• The Halogen is bonded to a carbon that is bonded to three other carbons
• These are usually found joined to the middle carbon of a carbon chain that also has a methyl group coming off the same carbon - i.e. Methylpropane

You can see diagrams of the three types below - when they react you get different products.

Nucleophillic Substitution

• halogens have a greater electronegativity than carbon

• electronegativity is the ability to attract the shared pair in a covalent bond

• a dipole is induced in the C-X bond and it becomes polar

• the carbon is thus open to attack by nucleophiles

• nucleophile means ‘liking positive’

Nucleophiles

• ELECTRON PAIR DONORS

• possess at least one LONE PAIR of electrons

• don’t have to possess a negative charge

• are attracted to the slightly positive (electron deficient) carbon

• examples are OH¯, CN¯, NH₃ and H₂O (water is a poor nucleophile)

Producing Alcohols from Halogenoalkanes

Reagent:

• Aqueous* sodium (or potassium) hydroxide

Conditions:

Reflux in aqueous solution (SOLVENT IS IMPORTANT)

Product:

Alcohol

Nucleophile:

hydroxide ion (OH¯)

Equation: e.g. C₂H₅Br(l) + NaOH(aq) ——> C₂H₅OH(l) + NaBr(aq)

Mechanism: