Physical properties of halogen compounds
Structure: Simple molecular structure, polar molecule
1. Higher MP and BP than alkanes with similar Mr (pdpd>idid)
2. BP increases down the group
(electron cloud size increases)
3. insoluble in water (pdpd between H2O and C-X molecule < pdpd between C-X molecules + hydrogen bonds between H2O molecules)
4. Soluble in organic solvents (idid between organic solvent and C-X molecule > idid between organic solvent molecules and pdpd between C-X molecules)
Explain why iodoalkanes have the highest boiling point compared to
bromoalkanes, chloroalkanes and fluoroalkanes with the same alkyl chain.
- Iodoalkanes have the largest electron cloud size.
- The electron cloud of the iodoalkanes is the most easily polarised.
- The iodoalkane contains stronger idid interactions between their molecules.
- Hence, more energy is needed to overcome the interactions in iodoalkane
Synthesis of halogenoalkanes
- FRS of alkanes
- Electrophilic addition of alkenes
- Nucleophilic substitution of alcohols
Electrophilic addition of alkenes
Dry HCl/ HBr/ HI(g), room temperature
OR
Cl2 in CCl4 / Br2 in CCl4, dark room, room temperature
Nucleophilic addition of alcohols
- PCl5, room temperature
- PCL3, heat under reflux
- SOCl2, heat under reflux
- Concentrated HCl, Zn catalyst, heat under reflux
- PBr3, heat under reflux
- NaBr3(s)/KBr(s), concentrated H2SO4, heat under reflux
- PI3, heat under reflux
- NaI(s)/KI(s), concentrated H2SO4, heat under reflux
Nucleophilic substitution of halogenoalkanes ( formation of alcohol)
NaOH/KOH(aq), heat under reflux
Nucleophilic substitution of halogenoalkanes (formation of nitriles)
Ethanolic NaCN/KCN, heat under reflux
(one of the two step-up reactions)
Nucleophilic substitution of halogenoalkanes ( formation of amines)
Mono-sub: Limited RX groups, excess concentrated NH3 in ethanol, heat in sealed tube
Multi-sub: Excess RX group, limited concentrated NH3 in ethanol, heat in sealed tube
Elimination to form alkenes
Ethanolic NaOH/KOH, heat under reflux (synthesis of alkenes)
* must have at least one H beside the C-X bond
Reaction of nitriles
- Hydrolysis
- Reduction
Hydrolysis of nitriles and its products
Acidic hydrolysis:
H2SO4/HCl(aq), heat under reflux
Product: CN–> COOH + NH4+
Basic hydrolysis:
NaOH(aq), heat under reflux
Product: CN–> COO- +NH3
Reduction of nitriles
LiAlH4 in dry ether or H2 gas, Ni catalyst heat
Product: amine
SN2 mechanism
- primary RX/ secondary RX
- one-step reaction
- Factor: steric hindrance
- Rate equation: k[RX][Nu]
Why primary RX undergoes SN2?
(Steric hindrance)
- primary RX has less bulky alkyl groups bonded which will less likely hinder the nucleophile’s approach, thus facing less steric hindrance
SN1 mechanism
- tertiary RX/ secondary RX
- two-step reaction
- Factor: Stability of carbocation
- Rate equation: K[RX]
Why tertiary RX undergoes SN1?
- tertiary RX has more e-donating alkyl groups
- positive charge on carbocation dispersed to a larger extent
- 3 degree carbocation more stable
Why halogenoarenes do not undergo nucleophilic substitution?
- Partial double bond character of the C-X bond
- The p-orbital of the halogen atom can overlap with the pi electron cloud of the benzene ring.
- The lone pair of electrons on Cl can be delocalised into the benzene ring.
- This results in the C–X bond having a partial double bond character which makes the C–X bond stronger and harder to break. - Interelectronic repulsion of nucleophile
- Nucleophiles are electron-rich species which will face interelectronic repulsion with the pi electron cloud of the benzene ring.
- Benzene provides high steric hindrance.
Distinguishing test for for RX
- Add NaOH(aq) and heat –> nucleophilic sub to release halide ion
- Add excess dilute HNO3 after cooling –> neutralise any excess base (OH-) to prevent AgOH ppt from forming
- Add AgNO3(aq) –> form ppt
White ppt AgCl –> chloroalkane
Cream ppt AgBr –> bromoalkane
Yellow ppt AgI –> Iodoalkane
