3.3.3 Halogenoalkanes - Chemistry Alevel

F prefix

Fluoro-

Cl prefix

Chloro-

Br prefix

Bromo-

I prefix

Iodo-

Halogenoalkane functional group description

A halogen atom attached to a carbon atom. NOTE: they contain polar bonds

Halogenoalkane general formula

R-X where R is the alkyl chain and X the halogen atom.

Synthesis of halogenoalkanes - word equation

Alkane + Halogen → Halogenoalkane + Hydrogen Halide

Synthesis of halogenoalkanes -conditions

UV radiation present or above 300°C

Synthesis of halogenoalkanes - reaction type

Substitution reaction.

Reactivity of alkanes

The alkanes and cycloalkanes, except cyclopropane, are the least reactive organic compound.

Alkanes reactivity reason

Alkanes contain strong carbon-carbon single bonds and strong carbon-hydrogen bonds as they have similar electronegativity and so are non-polar and cannot attract other molecules.

How can alkanes and halogens react.

Without the presence of UV light the halogen is not attracted to the alkane but in the presence of UV light the molecules have more energy, 400kJ mol⁻¹, and so they move around more.

Why are alkanes not susceptible to electro/nucleophile attack

C&H have very similar electronegativities so they are non-polar, there are no double bonds and so there are no electron rich regions so they can't attract electro/nucleophiles.

CFCs uses & reason for variety of uses

They are used as solvents and refrigerants as they are very unreactive. But now chemists have developed alternative Cl free compounds.

Why do CFCs cause problems?

Due to their unreactive quality when they leak into the atmosphere (either during use as cleaning solvents or from abandoned refrigerators) they persist for a very long time and gradually rise up to the higher atmosphere.

How do CFCs in the atmosphere cause a problem?

CFCs act as Greenhouse gases (contributing to global warming) and speed up the break down of the Ozone layer.

Why are CFCs good solvents?

As CFCs have high electronegativities so they are polar like water and are good solvents.

What is ozone made of?

Three atoms of oxygen bonded together to form a molecule with the formula O3.

Where is ozone in the atmosphere?

A small amount of ozone concentrated in the upper atmosphere (the stratosphere).

What are the benefits of ozone?

It acts like a protective shield, absorbing almost all of the ultraviolet radiation from the sun. And UV radiation leads to increased incidences of sunburn, ageing of the skin and skin cancer.

CFC effect on the homolytic fission of ozone molecules

C-Cl breaks due to the UV radiation making Cl radicals which act as a catalyst in the reaction as in remains unchanged at the end/it is regenerated.

Nucleophile - definition

An electron pair donor.

Nucleophilic substitution reaction - definition

A type of reaction where the halogen atom is replaced by another atom or group of atoms. 2 reactants → 2 products.

What does nucleophilic substitution form?

A compound with a different functional group and ∴ different chemical properties.

Why can halogenoalkanes undergo nucleophilic substitution?

Halogens are more electronegative than C and so the C-X bond is polar and can be attacked by nucleophiles which donate a pair of electrons to the C which is electron deficient.

Halogenoalkane polarity - group

The halogens become less electronegative down the group and so the polarity of the C-X bond decreases from F to I.

Cl - precipitate colour

White precipitate

Br - precipitate colour

Cream precipitate

I - precipitate colour

Yellow precipitate

Hydrolysis reaction base equation

CH3CH2X + OH⁻ → CH3CH2OH + X⁻

Reaction with halide and silver - half equation

Ag⁺(aq) + X⁻(aq) → AgX(s)

I - precipitate name

Silver Iodide AgI

Br - precipitate name

Silver Bromide AgBr

Cl - precipitate name

Silver chloride AgCl

I - speed of precipitation

Fastest

Br - speed of precipitation

Middle

Cl - speed of precipitation

Slowest

I - reactivity

Highest

Br - reactivity

Middle

Cl - reactivity

Lowest

I - bond polarity

Less polar

Br - bond polarity

Middle

Cl - bond polarity

More polar

I - bond enthalpy

238

Br - bond enthalpy

276

Cl - bond enthalpy

338

What does the reactivity of a halogenoalkane depend on?

The reactivity of the halogenalkanes depends on the strength of the C-X bond. The halogenoalkane with the weakest C-X bond will have the fastest rate of reaction.

Primary halogenoalkane - mechanism

Nucleophilic substitution

Secondary halogenoalkane - mechanism

Nucleophilic substitution and some elimination.

Tertiary halogenoalkane - mechanism

Elimination

Base elimination mechanism conditions

Ethanol (ethnic or alcoholic), high temperature, OH⁻ acts in elimination.

Nucleophilic substitution mechanism conditions

Water (aqueous), warm, OH⁻, CN⁻ or NH₃ act as a nucleophile.

Nucleophilic substitution background - OH⁻

The attacking group is a nucleophile, OH⁻ ions, which are attracted to the slightly positive carbon atom and have a lone pair of electrons which are donated to form a new covalent bond. The polar C-Br breaks heterolytically with Br getting both electrons from the bond. There has been a substitution (of a Br for a -OH group) and a hydrolysis reaction (only for OH⁻).

Nucleophilic substitution background - CN⁻

The attacking group is a nucleophile, CN⁻ ions, which are attracted to the slightly positive carbon atom and have a lone pair of electrons which are donated to form a new covalent bond. The polar C-Br breaks heterolytically with Br getting both electrons from the bond. There has been a substitution (of a Br for a -CN group).

Nucleophilic substitution background - NH₃

The attacking group is a nucleophile, NH₃ ions, which are attracted to the slightly positive carbon atom and have a lone pair of electrons which are donated to form a new covalent bond. The polar C-Br breaks heterolytically causing the bromide ion to leave the group. A second molecule of ammonia acts as a base, removing a proton from the positively charged intermediate. There has been a substitution (of a Br for a -NH₂ group).

Nucleophilic substitution uses - CN⁻

It is one of the ways of extending the length of the carbon chain.

Base elimination background

The hydroxide ion, OH⁻, acts as a base and removes a proton. The proton comes from a carbon atom adjacent to the one bonded to the bromine. The remaining electron pair forms a second bond between the carbon atoms. The bromide ion takes the pair of electrons from the C-Br bond. There is an elimination (of HBr from 2-bromopropane).