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Haloalkanes Notes

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Unit 2: 14 Haloalkanes

HALOALKANES Why are alkanes so unreactive?

• Both C-C bonds and C-H are non-polar therefore particles are not attracted to the alkanes

• Both C-C and C-H bonds are very strong Being so unreactive makes alkanes useful as LUBRICANTS and PLASTICS

FREE-RADICAL SUBSTITUTION

A FREE RADICAL is a species with an unpaired electron There are 3 ways to split the shared electron pair in unsymmetrical covalent bond; HETEROLYTIC FISSION - unequal splitting of electrons so both electrons go to one atom (produces ions) HOMOLYTIC FISSION (equal splitting) occurs when a covalent bond breaks with equal splitting of the bonding pair of electrons, to produce free radicals.

SYNTHESIS OF HALOALKANES - FREE-RADICAL SUBSTITUTION MECHANISM

STEP 1: INITIATION STEP UV •

Cl2 → Cl + Cl

• UV LIGHT provides the energy needed to start the reaction by splitting chlorine molecules into free radical atoms

• This process occurs first because the Cl-Cl bond in chlorine is weaker than the C-H bond in methane

STEP 2 & 3: PROPAGATION STEPS FIRST PROPAGATION STEP

CH4 + Cl → CH3 + HCl SECOND PROPAGATION STEP

CH3 + Cl2 → CH3Cl + Cl

Chlorine radical is regenerated - acting as a catalyst

-Chloromethane(HALOALKANE)

• In each propagation step, a radical is used and a radical is form - continuous chain reaction

OVERALL EQUATION

UV

CH4 + Cl2 → CH3Cl + HCl TERMINATION STEPS

• CH3 + • CH3 → C2H6

• CH3 + • Cl → CH3Cl

-Such termination steps lead to trace amount of impurities in the final products.
-You can begin the mechanism with a larger molecule to achieve a larger molecule (READ THE QU.) CHEMISTRY AS NOTES

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