When one or more halogen atoms (X = F, Cl, Br , I) bonds with carbon atoms alkyl halides forms.
There may be single bonds, double bonds, triple bonds between carbon atoms in alkyl halides.
Aliphatic or aromatic hydrocarbons which are halogen substitited are defined as alkyl halides. In this tutorial, we will lean about physical and chemical properties of alkyl halides.
Some examples are CH3CH2Cl, CH3Br , CH3CH2I.
CFC (Freon), HFC compounds which are used now in refrigerators instead of CFC are alkyl halides.
A/C gas - 1,1,1,2-tetrafluoroethane
When a halogen atom is connected directly to benzene ring, they are called aryl halides. py orbital of halogen interacts with benzene ring. Thus that C-X bond shows partial double bonds characteristics. Due to partial double bonds characteristics, that C-X bond is the strongest C-X bond in organic compounds. Therefore, aryl halides show a special inert behaviour and differ from the typical aliphatic alkyl halide compounds reactions.
Alkyl halide can be prepared by the reaction of halogen acids with alcohols. Hydrochloric (HCl), hydrobromic acid (HBr),
Hydrogen iodide acid (HI) are halogen acids. Alcohols react with these acids and give alkyl halides. In this reaction
, a halogen is substituted instead of OH.
ROH + HX → RX + H2O
NOTE: To occur the reaction easily with HCl, a catalyst ZnCl2 should be used.
Alkyl chlorides are formed by the reaction of alcohols and inorganic acid chlorides. Phosphorus(III) chloride (PCl3)
or Phosphorus(V) chloride (PCl5) or thionyl chloride (SOCl2) are used as inorganic acid chlorides.
ROH + PCl3 → RCl + H3PO3
ROH + PCl5 → RCl + POCl3 + HCl
ROH + SOCl2 → RCl + SO2 + HCl
Carbon halogen bond (C-X) is polarized in alkyl halides. Therefore carbon atom gets a small positive charge(δ+). Hence,
react with 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 reaction. When eliminated aqueous halide anion becomes more stable, the reaction gets easier to occur.
Alkenes are prepared by the reactions of alcoholic alkali(NaOH, KOH) and alkyl halides with 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.
Alcohols are prepared by the reaction of alkyl halides and aqueous alkali. The halogen group is substituted by hydroxyl ion. In
this reaction hydroxyl group behaves as a nucleophile.
Magnesium and dry ether is mixed with alkyl halide to prepare
For keep Grignard stable, dry ether is essential. These Grignard reagents are used to extend carbon chains. In aqueous medium, grignard reagent slowly hydrolyzes and give hydrocarbons.
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.
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.
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.
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.
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.
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
Freons are used in refrigerators. Freons are fluoro halohydrocarbon such as CCl3F, CCl2F2.
CHBrClCF3 is used as an anesthetic.
In these examples, we use our knowledge about alkyl halides, alkane, alkene, alkyne. So you can get remembered previous lessons.
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 wwith aqueous HCl. To connect a chlorine atom, you have to use anhydrous ZnCl2/ concentrated HCl mixture or PCl3 or PCl5.
You have learnt 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 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.