alkanes and haloalkanes

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27 Terms

1

nucleophilic substitution mechanism definition

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

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2

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)

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3

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)

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4

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

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5

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

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6

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

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7

Initiation step in free radical substitution

-UV radiation breaks a covalent bond

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

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8

propagation step

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

-Cl. + CH4 => HCl + .CH3

-.CH3 +Cl2 => CH3Cl + Cl.

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9

termination step

2 free radicals combine to make a stable molecule

-Cl. + .CH3 => CH3Cl

-Cl. +Cl. => Cl2

-.CH3 +.CH3 => C2H6

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10

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

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11

Homolytic fission

covalent bond breaks evenly

one e- from bond goes to each atom

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12

use of haloalkanes

solvents, chemical feedstocks, propellants

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13

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

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14

steps of UV breaking CFC by homolytic fission

CCl3F => Cl. + .CCl2F

.Cl + O3 => .ClO +O2

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

-overall: 2O3 => 3O2

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15

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

<p>-dipole on C-I/F/Cl , c is delta +,</p><p>-curly arrow from bond to delta - halogen</p><p>-curly arrow from electron pair on OH- to C delta +, forming dative covalent bond</p><p>-alcohol + halide formed</p><p><span style="color: blue">-boiled under reflux with aqueous KOH (alkali) solution</span></p>
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16

nucleophile

electron pair donor

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17

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+

<p>-water is nucleophile, curly arrow from e- pair on O to C delta +</p><p>-halide is delta -, curly arrow from C-halogen bond to halogen</p><p>-intermediate formed</p><p>-products are alcohol, halide, H+</p>
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18
<p>reflux</p>

reflux

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

<p><strong>heating the chemical reaction for a specific amount of time, while continually cooling the vapour produced back into liquid form, using a condenser</strong></p>
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19

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

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20

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

tetrahedral

4 bonding e- pairs repel equally

109.5

<p>tetrahedral</p><p>4 bonding e- pairs repel equally</p><p>109.5</p>
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21

what’s the reason for incomplete combustion of alkanes

insufficient oxygen supply, making CO

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22

what tests for halogens

silver bromide

then nitric acid

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23

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

silver nitrate test

time takes to form precipitate

Ag+ + Cl- => AgCl (s)

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24

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

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

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25

why is water a weaker nucleophile than a hydroxide ion

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

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26

breakdown of ozone by NO radicals

NO. + O3 = NO2 + O2

NO2 + O. = NO. + O2

overall: O3 + O = 2O2

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27

limitations of free radical substitution

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

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