alkanes and haloalkanes

nucleophilic substitution mechanism definition

-involves a nucleophile, where an atom in a molecule is replaced by another

CFCs

chlorofluorocarbons

harmful to ozone layer owing to release of chlorine atoms on exposure to UV radiation

UV breaks C-Cl bond, forming a Cl. free radical, which destroys ozone molecules, causing hole in ozone layer

uses: propellant in aerosol sprays (volatile), dry cleaning (non-toxic and good solvents)

why are alkanes unreactive

C-H bond is short, strong as nuclei of both atoms are close to shared pair of electrons

and non-polar so other reagants aren’t attracted to them

C-C and C-H have sigma bonds (overlap of s-orbitals directly between nuclei)

free radical substitution

alkanes react with halogens in presence of UV light

involves free radicals where atom in a molecule is replaced by another

photochemical reaction brought about by light

homologous series

a series of organic compounds with the same functional group, and therefore similar chemical properties. Each successive member differs by CH2.

There’s a gradual change in physical properties due to increasing molecular size

trends in increasing size of alkane molecules

-boiling point and viscosity increase, molecule gets bigger, number of electrons per molecule increases, induced dipole forces get stronger (more contact surface area), more energy required to break them

-flammability and volatility decrease, less likely to evaporate

Initiation step in free radical substitution

-UV radiation breaks a covalent bond

-CL2 => 2Cl. (2 chlorine free radicals)

propagation step

free radicals are simultaneously consumed and regenerated during the course of a chain reaction

-Cl. + CH4 => HCl + .CH3

-.CH3 +Cl2 => CH3Cl + Cl.

termination step

2 free radicals combine to make a stable molecule

-Cl. + .CH3 => CH3Cl

-Cl. +Cl. => Cl2

-.CH3 +.CH3 => C2H6

why do branched molecules have a lower melting point

-straight chained can pack more tightly together

-branched have less contact surface area

-weaker induced dipole forces

Homolytic fission

covalent bond breaks evenly

one e- from bond goes to each atom

use of haloalkanes

solvents, chemical feedstocks, propellants

why don’t alkanes mix with/dissolve in water

hydrogen bonds need to be broken and form induced dipole forces which are weaker and don’t compensate for hydrogen bonds

steps of UV breaking CFC by homolytic fission

CCl3F => Cl. + .CCl2F

.Cl + O3 => .ClO +O2

.ClO + O3 => .Cl (regenerated) + 2O2

-overall: 2O3 => 3O2

Nucleophilic substitution mechanism

-dipole on C-I/F/Cl , c is delta +,

-curly arrow from bond to delta - halogen

-curly arrow from electron pair on OH- to C delta +, forming dative covalent bond

-alcohol + halide formed

-boiled under reflux with aqueous KOH (alkali) solution

nucleophile

electron pair donor

hydrolysis of haloalkanes

-water is nucleophile, curly arrow from e- pair on O to C delta +

-halide is delta -, curly arrow from C-halogen bond to halogen

-intermediate formed

-products are alcohol, halide, H+

reflux

heating the chemical reaction for a specific amount of time, while continually cooling the vapour produced back into liquid form, using a condenser

why is C-I bond weaker than C-Br, so iodoethane is more reactive than bromoethane

it’s weaker because it’s longer

because I atom is bigger so nucleus further from e- pair, less nuclear attraction

what’s the shape and bond angle around a C in an alkane

tetrahedral

4 bonding e- pairs repel equally

109.5

what’s the reason for incomplete combustion of alkanes

insufficient oxygen supply, making CO

what tests for halogens

silver bromide

then nitric acid

how would you monitor rate of reactions of iodo, bromo and chloroethane

silver nitrate test

time takes to form precipitate

Ag+ + Cl- => AgCl (s)

why are hydrochlorofluorocarbons (HCFCs) kinder to the environment than CFCs

contain hydrogen, break down more easily in atmosphere, less harmful to ozone layer

why is water a weaker nucleophile than a hydroxide ion

-doesn’t have as many lone pairs or a negative charge

breakdown of ozone by NO radicals

NO. + O3 = NO2 + O2

NO2 + O. = NO. + O2

overall: O3 + O = 2O2

limitations of free radical substitution

-there can be further substitutions (if chlorine radicals react with products)