Halogenoalkanes

What are Halgoenoalkanes?

Halogenoalkanes are also known as haloalkanes or alkyl halides. Simply put, they are alkanes, but they contain a halogen atom instead of one (or more!) of their hydrogen atoms. The halogen atom is referred to as X. The general formula is CnH2n+1

Reactivity of halogenoalkanes compared to alkanes

Halogenoalkanes are generally more reactive than alkanes due to the presence of a halogen atom, which is more electronegative than carbon. This results in a polar C-X bond (where X is the halogen), making it susceptible to nucleophilic attack.

What is nomenculture (halogenoalkanes)

Straight chain halogenoalkanes are named according to the parent alkane. The longest continuous straight chain of carbon atom is selected and numbered in order to indicate the position of the halogen substituent on the chain. The end of the chain starts is chosen so as to use the lowest values. The name of the halogen prefixes the name of the parent alkane.

What are the classes of halogenoalkanes

They can be classified as primary (1^o) secondary(2^o) tertiary (3^o) according to their structure

How does an alkane reaction with chlorine to form a halogenoalkane

Alkanes react with chlorine explosively in the presence of ultra violet light

Why would there no reaction if a reaction to make a halogenoalkane was not done use UV light

The reaction in the presence of UV light is very fast as it takes place via a free radical substitution mechanism. The reaction has a number of distinctive steps:

What is a free radical?

A species with an unpaired electron

What are the 3 steps involve in a reaction between an alkane and halogen

Initiation step. propagation step and termination step

What is the initiation step (Chlorine and methane)

This step involves the breaking of the Cl-Cl covalent bond forming 2 Cl atoms (or radicals) by exposing the chlorine molecule to UV light. The Cl-Cl bond breaks as it is the weakest bond.

Propagation step (Chlorine and methane)

This step involves the chlorine atom attacking a methane molecule forming a methyl radical. The methyl radical further reacts with another chlorine molecule forming chloromethane and another Cl atom. Hence the chain is propagated

Termination step (Chlorine and methane)

The reaction is terminated whenever radicals are removed from the reaction, it is very easy for such a fast reaction as this to quickly build up a mixture of products. The chloromethane is also reactive towards chlorine radicals and is quickly converted into dichloromethane. In order to limit a further reaction, an excess of CH₄ is used.

What is always formed during the first propagation step

A hydrogen halide

What are the uses of halogenoalkanes

They can be used as solvents as they tend to be unreactive liquids which are good at dissolving other polar organic compounds. The chlorofluoroalkanes are often called CFCs ( chloro-fluoro carbons) and widely used as the coolant in fridges.

Why is ozone good?

Ozone formed naturally in the upper atmosphere is beneficial as it absorbs harmful ultra-violet radiation which damages cells and can cause skin cancer.

How did halogenoalkanes build up in the atmopshere, what did it cause and why is this a problem?

The chlorofluoroalkanes are very unreactive and do not dissolve in water so they diffused gradually over many year up into the upper atmosphere. Here the ultra violet light decomposed the molecule forming chlorine atoms as the energy broke the C-Cl bonds. The chlorine atoms catalyse the decomposition of ozone and contributed to the formation of a hole in the ozone layer

How does electronegativity work in halogenoalkanes

The halogen atoms are much more electronegative than the C-atom (with exception of I) and, therefore, the pair of electrons forming the C-X bond is displaced towards the halogen. Hence, the bond is polarised and has some ionic character.

What influences the rate of reaction in a halogenoalkane

The strength of the carbon -halogen bond

Why are fluoroalkanes slow to react and how does the reactivity increase between the halogens?

Although the C-F bond is very polar it is very strong so fluoroalkanes are slow to react. Reactivity increases as follows:

CH₃F<CH₃CL<CH₃Br<Ch₃I

What are nucleophiles classed as?

They are classified as electron pair donors,; they are negatively charged ions or compounds in which an atom has a lone pair of electrons

How does nucleophilic substiution work?

Nucleophilic substitution occurs when a nucleophile, an electron pair donor, attacks a carbon atom in a halogenoalkane, which is bonded to a halogen (the leaving group). The nucleophile replaces the halogen in a two-step process

What are the 2 steps of nucleophilic substitution

1. Formation of a carbon-nucleophile bond: The nucleophile approaches the carbon, forming a temporary bond while the C-X bond begins to break.

2. Leaving group departure: The halogen, now with an extra pair of electrons, leaves as a halide ion, completing the substitution. This results in the formation of a new compound with the nucleophile attached to the carbon.

Why is an electron pair displaced towards the C atom in the C-X bond

The halogen atoms are much more electronegative than the C-atom (with the exception of I) and, therefore, the pair of electrons forming the C-X bond is displaced towards the halogen. Hence, the bond is polarised and has some ionic character.

What is the rate of reaction influenced by in halogenoalkanes

The strength of the carbon-halogen bond. As the halogen gets larger the bond strenght decreases, greater distance and shielding between nucleus and shared pair of electrons so weaker attraction

Reactivity pattern of halogenoalkanes

CH₃F< CH₃Cl<CH₃Br<CH₃I

What do nucleophilc reagants attack in terms of Halogenoalkanes

they attack the electron deficient end of the C-X bond. The halogenoalkanes are susceptible to he nucleophilic attack due to the C^s+.

What substance/compound is used during nucleophilic substitution normally

warm aqueous sodium hydroxide

Nucleophilic substitution image and explanation

The curly arrows represent the movement of electron pairs. The :OH^- acts as a nucleophile attacking the slightly positive C-atom and donating its lone pair of electrons. The mechanism shows that a C-OH bond is formed and at the same time the C-Br bond breaks. The reaction results in the overall substituion of an OH group for a -Br atom. So it is a nucleophilic substituiton reaction

What does nucleophillic substitution involving sodium hydroxide always produce?

An alcohol

What raegents are used in the nucleophilic substitution between cyanide ions

KCN or NaCN

Nucleophilic substitution with cyanide image

fully dissociates to :CN ions

What do these reactions from cyanide always form?

A nitrile

Why is nucleophilic substitution with cyanide useful in industry

Carbon chain increases by one

Nucleophillic substituion with aqueous ammonia

CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ =NH₄Br

(bromoethane) +(excess ammonia) → (ethylamine) +(ammonium bromide)

Why is excess ammonia normally used with nucleophilic substitution

To minimises the chance of further reactions producing secondary and tertiary amines.

What is used to represent the movement of electron pairs

curly arrows

Nucleophillic substiution mechanism with ammonia

furthest to the right nitrogen has a lone pair of electrons

2nd NH3 attack explained in nucleophillic substitution

The NH₃ molecule possesses a lone pair of electrons on the N atomn and hence, acts as a nucleophile. As the N is providing both electrons to form the new C-N bond, it acquires a positive charge. The HBr released reacts with NH₃ to form NH₄Br.

Why is nucleophillic substitution with NH₃ a poor method to make amines industrially.

Further substitution ocurrs which leads to a mixture of products forming which have to be separated

What can we do to make amines efficiently

When using nucleophilic substitution use excess NH₃ because it minimizes the chance of separation having to occur. Use a nitrile intermediate instead (beware it is toxic) you can slo use reduction/ hydrogenation

Mechanism for Elimination

Explanation of elimination mechanism

The lone pair on O forms coordinate bond with H this forces shared pair of electrons on C-H bond to form C=C. This repels electrons from C-Br bond onto Br and C-Br bond break

Which classes of halogenoalkanes are favoured but substitution or elimination

Primary → Substitution

Secondary → depends on conditions

Tertiary → depends on conditions

When can you only get elimination?

With a strong base acting as the nucleophile. (NaOH/ KOH) (Not NH3)

How can you make NaOH a stronger base?

By dissolving it

What temperatures favour substitiution/ elimination

Low temperatures favours substitution

High temperatures favour elimination

Overall what conditions are elimination reactions favoured by

Hot, ethanoic, KOH or NaOH and tertiary structures

Overall what are substitution reactions favoured by?

Warm aqueous, KOH or NaOH