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.
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.
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.
Alkyl halide show so many reactions and we will discuss how these reactions are occurred.
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.
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
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
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
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.
Reaction rate of alkyl halides depends on whether alkyl halide is a primary or secondary or tertiary and type of halogen. ( Cl, Br, I ).
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.
We consider primary, secondary and tertiary alkyl halides with same halogen. This reaction rate also depends on two factors.
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.
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).
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.
When but-1-ene reacts with hydrogen halide such as HBr, it forms 2-bromobutane. This is a secondary alkyl halide.
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.
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.
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.
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.
When alcoholic aqueous potassium cyanide(KCN) add to alkyl halide, halogen group(-Cl, -Br, -I) is substituted by a -CN group. We can prepare bromoethane(CH3CH2Br) by reaction of ethanol and PBr3. Otherwise we first prepare ethene(CH2CH2) from ethanol and then bromoethane is prepared from ethene. Then alcoholic aqueous potassium cyanide(KCN) is added to bromoethane to get cynoethane.