Chemistry - 3.3.3: Halogenoalkanes

Halogenoalkanes

Hydrocarbons with an alkane skeleton with one or more halogen atoms in place of hydrogen atoms

General formula of halogenoalkanes and the shortened formula

CnH2n+1X
R-X

4 prefixes for halogenoalkanes (fluorine, chlorine, bromine, iodine)

Fluoro-, chloro-, bromo-, iodo-

3 physical properties of halogenoalkanes

1. Polarity: C-X bond is polar due to difference in electronegativity - as you go down the group, bonds become less polar (has both VdW and permanent dipole-dipole forces)
2. Boiling point: increases with chain length + as you go down group 7, decreases with chain branching
3. Solubility; Insoluble in water but can mix with other hydrocarbons

Symbol for halogens in general formulae

X

Nucleophile

An ion/group of atoms that donate electrons to form a covalent bond (either negatively charged ions and/or molecules with a lone group of electrons)

Electrophile

An ion/group of atoms that accepts electrons to form a covalent bond (either positively charged ions and/or do not have a full outer shell of electrons)

What do nucleophiles have?

A lone pair of electrons

3 common nucleophiles

:OH-, :NH3 and :CN- (hydroxide ions, ammonia and cyanide ions)

Nucleophilic substitution

An organic reaction where a molecule with a partially positively charged carbon atom is attacked by a nucleophile, resulting in the original atom or group of atoms being replaced by the nucleophile

General equation for nucleophilic substitution

CH2RX + :Nu- -> CH2RNu + :X-

What do curly arrows show?

How electron pairs move in organic reactions

Leaving group

The atom/group of atoms ejected from the starting material after an organic substitution reaction, normally taking with it an electron pair and forming a negative ion

In nucleophilic substitution, what is the leaving group?

The halide ion

What affects the rate of a nucleophilic substitution reaction? Why?

The halogen - as you go down the group, C-X bond becomes weaker so less energy is needed to break it and the rate of reaction increases

What is formed in the nucleophilic substitution reaction between a halogenoalkane and a hydroxide ion?

An alcohol and a halide ion

What is formed in the nucleophilic substitution reaction between a halogenoalkane and a cyanide ion?

A nitrile and a halide ion

What is formed in the nucleophilic substitution reaction between a halogenoalkane and an ammonia molecule?

An amine and NH4X (ammonium halide)

Elimination reaction

A reaction where an atom/group of atoms is removed from a reactant

In elimination reactions, what is eliminated from the molecule?

A halide ion and a molecule of water

In elimination reactions, what is formed and why?

An alkene - a double bond forms after the hydrogen halide is eliminated

What happens in the elimination reaction where OH- acts as a base (3)?

- OH- uses lone pair, forms bond with one of hydrogen atoms on carbon of C-X bond as hydrogens have slight positive dipole, forms water (curly arrow from lone pair to H - H must be from a correct C-H bond adjacent to the C-X bond) - HYDROGEN AND BROMINE SHOULD BE BONDED TO ADJACENT CARBON ATOMS AND BOTH BE VERTICAL
- Electrons from C-H now used to form C=C (curly arrow from a correct C-H bond adjacent to the C-X bond)
- X takes pair of electrons from C-X, leaves as X- (curly arrow from C-X bond to X)
- Produces alkene + X- + H2O
- X- may then react with potassium (or sodium) if OH- is in potassium hydroxide, producing alkene + KX + H2O

Conditions for the elimination reaction where OH- acts as a base (3)

1. Ethanolic
2. High concentration of hydroxide
3. High temperature

What conditions and type of hydrocarbons favour elimination?

- Hydroxide ions at high temperature, dissolved in ethanol
- Tertiary and secondary hydrocarbons

What conditions and type of hydrocarbons favour substitution?

- Hydroxide ions at room temperature, dissolved in water (aqueous)
- Primary and secondary hydrocarbons

What is the reactivity of CFCs?

Unreactive at normal conditions

How do you name nitriles?

Alkanenitrile (include carbon in CN-)

Conditions for nucleophilic substitution with ammonia (4)

- Excess of ammonia
- Dissolved in ethanol / ethanolic
- Heated mixture
- High pressure

Conditions for nucleophilic substitution reactions with hydroxide ions (3)

- Warm / heat
- Aqueous / dissolved in water
- Dilute hydroxide

Why can halogenoalkanes undergo nucleophilic substitution?

The carbon-halogen bond is polar

Why can OH- / CN- act as nucleophiles?

They have a lone pair of electrons and a negative charge

What must happen for a neutral nucleophile to be used in nucleophilic substitution?

They must lose a proton

How does ozone in the upper atmosphere benefit life on Earth?

It absorbs harmful UV radiation

Outline the reaction mechanism for the nucleophilic substitution of a halogenoalkane with ammonia ions

1. Lone pair of electrons transferred from C atom to halogen atom (forms halide ion). NH3- ion transfers a lone pair of electrons to the same C, forming a bond
2. The N of the NH3 added to the molecule has a positive charge - another NH3- ion transfers a lone pair of electrons to one of the H of the added NH3, while the electrons of the N-H bond are transferred to the positively charged N atom (forms NH4+ ion and an amine)

Outline the reaction mechanism for the nucleophilic substitution of a halogenoalkane with hydroxide ions

Lone pair of electrons transferred from C atom to halogen atom (forms halide ion) + lone pair of electrons from OH- transferred to same carbon atom (forms alcohol)

Outline the reaction mechanism for the nucleophilic substitution of a halogenoalkane with cyanide ions

Lone pair of electrons transferred from C atom to halogen atom (forms halide ion) + lone pair of electrons from CN- transferred to same carbon atom (forms nitrile)

What is the role of hydroxide ions in nucleophilic substitution?

Nucleophile

What is the role of hydroxide ions in elimination reactions?

Base

Why do some molecules (e.g. CO2 or trifluoroethane) absorb infrared radiation? (2)

1. The bonds vibrate
2. The bonds are polar

What causes global warming?

When a molecule has polar bonds and they vibrate at the same frequency as infrared radiation, so absorbs infrared radiation

Conditions for nucleophilic substitution reactions with cyanide ions

Ethanolic, heated under reflux

Order the following halogenoalkanes by their rate of hydrolysis (fastest → slowest) and explain why.

1-chlorobutane, 1-bromobutane and 1-iodobutane.

Iodo, bromo then chloro: C-I bond is weakest, then C-Br; C-Cl is strongest (carbon-halogen bond strength decreases as you go down the group)