Physical and Chemical Properties of Alkyl halides | Reactions

Alkyl halide compounds are defined when hydrogen atom(s) in a hydrocarbons is (are) substituted by halogen atom(s). There may be single bonds, double bonds, triple bonds between carbon atoms in the carbon chain. Due to the presence of halogen atoms, there are specific characteristics in these alkyl halides. Alkyl halides can be classified as Aliphatic alkyl halides and aromatic alkyl halides. In this tutorial, we will lean about physical and chemical properties of alkyl halides with reactions.

Content

Aliphatic alkyl halides and aryl alkyl halides
Physical properties of alkyl halides
- Solubility in water and other solvents
- Physical states at room temperature
- Melting and boiling points
Chemical properties and reactions of Alkyl halides
- Nucleophilic substitution reactions of alkyl halides
- Reaction rate of nucleophilic substitution reactions of alkyl halides
- Preparing alkenes by alkyl halides
Uses of alkyl halides due to specific chemical properties

Aliphatic alkyl halides and aryl alkyl halides

There are similarities and differences of aliphatic alkyl halides and aryl alkyl halides in physical properties and chemical characteristics. Aryl halide compounds are defined as when halogen atom(s) is (are) diretly joint with the carbon atoms of benzene ring.

Physical properties of alkyl halides

Solubility in water and other solvents

Alkyl halide compounds are categorized as non-polar compounds. Therefore, All alkyl halides are insoluble in water. But they dissolve in non-polar organic solvents such as CCl₄, Benzene, hexane.

Physical states at room temperature

Physical state of a chemical depends on the molecular weight and the strength of intermolecular forces between molecules. First members in alkyl halide series are in gaseous state due to less molecular weight, but alkyl halides with high molecular mass exist as liquids.
So, simple alkly halides as bromoethane, chloromethane exist as gases at room temperature.

Melting and boiling points

As physical state of a compound depend on molecular weight and the strength of intermolecular forces, melting and boiling point also vary on such factors.
C-X bond is polarized because electronegativity of halogens are higher than carbon atom. Therefore, there are week bipolar attractions between molecules. This results, having higher melting and boiling points for alkyl halides than corresponding molecular masses of alkanes.
melting and boiling points increase with molecular mass.
When halogen changes from Fluorine to Iodine, boiling and melting points are increased due to rise of molecular mass.

Chemical properties and reactions of Alkyl halides

Alkyl halide show so many reactions and we will discuss how these reactions are occurred.

Nucleophilic substitution reactions of alkyl halides

Carbon halogen bond (C-X) is slightly polarized in alkyl halides. Therefore carbon atom gets a small positive charge (δ+) and Halaogen atom receives a small positive charge (δ). Hence, nucleophilic reagents prefer to attack that carbon atom.

When nucleophile attacks to the carbon atom, halogen atom is removed as a halide ion (X^-). Therefore these reactions are defined as nucleophilic substitution reactions.

Examples for nucleophilic substitution reactions for Alkyl halides

Reaction of aqueous dilute alkaline( NaOH, KOH) and alkyl halides
Reaction of aqueous cyanide (CN^-) ions and alkyl halides

Prepare alcohols by alkyl halides

Alcohols are prepared by the reaction of alkyl halides and aqueous alkali. The halogen group is substituted by a hydroxyl ion. In this reaction hydroxyl group behaves as a nucleophile and attack the slightly positive carbon atom.

Ethyl bromide and aqueous KOH

Ethyl bromide reacts with aqueous KOH solution and give ethanol as the product. As a by-product, Potassium bromide is given.

1-bromopropane and aqueous KOH

Prepare Grignard reagent by alkyl halides

Magnesium and dry ether is mixed with alkyl halide to prepare Grignard reagent. For keep Grignard reagent stable, dry ether is essential. These Grignard reagents are used to extend carbon chains. In aqueous medium, grignard reagent slowly hydrolyzes and give hydrocarbons.

Bromoethane, Mg and dry ether

Bromobenzene, Mg and dry ether

Grignard reaction and alkyl halide reaction

Alkyl halide and grignard reagent react to give alkanes. This reaction can be used to extend carbon chain in organic chemicals synthesis.

Ethyl bromide and grignard reaction

Reaction of aqueous alcoholic cyanide ion solution and alkyl halide

Cyanide ion will replace the halogen atom. In this reaction, cyanide atom behaves as the nucleophile. First, carbon - bromine bond breaks and electrons of the bond are removed with bromine atom as the bromide ion. New carbon - cyanide bond is formed by the electrons which are provided by cyanide ion.

Bromoethane and aqueous alcoholic potassium cyanide

Reaction of ammonia and alkyl halide

Amines are formed by the reaction of Alkyl halide and ammonia. According to the amount of reactants primary amine or secondary amine or tertiary amine or ammonium salt are produced. If you use excess ammonia and less alkyl halide, primary amine is produced. If you use excess alkyl halide and less ammonia, it will give tertiary amine, ammonium salt is prepared.

Excess ammonia and less alkyl halide

Primary amine or secondary amine or mixture of these two can be given as products by the reaction of excess ammonia and less alkyl halide.

Less ammonia and excess alkyl halide

Tertiary amine or ammonium salt or mixture of these two can be given as products by the reaction of excess ammonia and less alkyl halide. Produced secondary amine reacts with alkyl halide and produce tertiary amine. If there is more alkyl halide, produced tertiary amine again reacts with alkyl halide to give ammonium salt.

Mechanism of ammonia and alkyl halide reaction

In this reaction ammonia behaves as a nucleophile and a base. And ammonia is a neutral molecule. First, ammonia molecule attacks the carbon atom which is connected to the halogen group. Halogen - carbon (C-X) bond breaks and forms a carbon - nitrogen (C-N) bond to give ammonium salt. Here ammonia behaves as a nucleophile. Second, a proton(H) which is attached to the nitrogen atom is attacked by another ammonia molecule. That proton is removed by giving electrons of N-H bond to nitrogen to give primary amine. In second step behaves as a base. Also ammonium halide is given.

Ammonia attacks alkyl halide to prepare ammonium salt

Ammonia attacks ammonium salt to give primary amine

Reaction rate of nucleophilic substitution reactions of alkyl halides

Reaction rate of alkyl halides depends on whether alkyl halide is a primary or secondary or tertiary and type of halogen. ( Cl, Br, I ).

Reaction rate variation according to halogen in alkyl halides

C-X bond length is increased when moving down in the halogen group. With increase of bond length, bond energy is decreased. Increment of reaction rate is caused by this bond energy decreasing.

Reaction rate according to the primary or secondary or tertiary alkyl halides

We consider primary, secondary and tertiary alkyl halides with same halogen. This reaction rate also depends on two factors.

Stability of carbocation
Steric impediment on halocarbon

Stability of carbocation

Carbocations holds positive charges. Carbocations are formed as intermediate compounds in the reactions.
If positive charge density is low, that carbocation show a stability.
That positive charge density depends on number of other alkyl groups which are around positively charged C atom and state of those alkyl groups. Alkyl groups (CH₃CH₂- , CH₃- ) can release electrons.
Therefore alkyl groups can reduce positive charge of carbocation.
Also, when number of alkyl group increases, positive charge of carbocation decreases.

Steric impediment on halocarbon

To a reaction occur, two molecules should be collided with each other. So, molecules should come close to each other. If molecules cannot be reached closely enough, no reaction will be happened.

When number of alkyl groups are increased around halocarbon atom which is covered by those alkyl groups. Therefore ability of reaching of nucleophilic reagents to the halocarbon is reduced. So we understand now, when increment of number of alkyl groups, rate of reaction is decreased.

So tertiary alkyl halides have the lowest reaction rates and primary alkyl halide have the highest reaction rates.

Preparing alkenes by alkyl halides

Alkenes are prepared by the reactions of alcoholic base (NaOH, KOH) and alkyl halides with the supply of heating. Halogen atom in the alkyl halide and a hydrogen atom in adjacent two carbon atoms are removed in this reactions to give the alkene , H₂O and the salt (NaBr, KBr, NaCl).

Reaction of chloroethane, alcoholic KOH to prepare ethene

Reaction of chloroethane, alcoholic KOH to prepare propene

Reaction of 1-bromopropane, alcoholic KOH to prepare propene

If you use aqueous alkali instead of alcoholic alkali, halogen atom will be substituted by OH^- ion.

CH₃CH₂Br + aqueous KOH → CH₃CH₂OH + KBr

Increasing the temperature and hydroxyl ion concentration will increase the eliminating reaction to give alkenes.

Uses of alkyl halides due to specific chemical properties

Chloroform, methyl chloride are used as solvents. Tetrachloroethylene, trichlorofluoroethane are used for dry cleaning processes. Ethyl chloride is used to manufacture tetraethyl lead which uses as antiknock agent in petrol.
Freons are used as refrigerants in refrigeration purposes. Freons are fluoro halohydrocarbon such as CCl₃F, CCl₂F₂.
CHBrClCF₃ is used as an anesthetic.

Questions

does but-1-ene produce a tertiary alkyl halide when reacted with hydrogen halide?

When but-1-ene reacts with hydrogen halide such as HBr, it forms 2-bromobutane. This is a secondary alkyl halide.

Reaction not used to prepare alkyl halide from alcohol

Alcohol and aqueous HBr reaction
Alcohol and aqueous HCl reaction
Alcohol and PCl₅ reaction
Alcohol and aqueoues HI reaction

Alcohol does not react with aqueous HCl. To connect a chlorine atom, you have to use anhydrous ZnCl₂/ concentrated HCl mixture or PCl₃ or PCl₅.

Will halogen atom be eliminated as a halide ion from alkyl chloride molecules in water?

You have learned that there are three different kinds of alkyl halides. Carbocations of tertiary alkyl halides are much stable than primary and secondary alkyl halides. So halide is easily removed in tertiary alkyl halide molecule in the water and give respective alcohols.

Are alkyl halides soluble in water?

Alkyl halides have some polarity. But alkyl halides can't form hydrogen bonds with water. Therefore alkyl halides are not soluble organic compounds in water.

Prepare cynoethane from ethanol

You have to synthesis cynoethane from two ways using ethanol as the starting organic compound. Fill all blank squares with required reagents and product of each different reaction.