alkanes

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32 Terms

1

alkane

a homologous series made up of saturate hydrocarbons which are joined by sigma bonds

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2

what ways can alkanes form

plankton die, fall to the ocean floor→ covered in layers of mud→ fossils over a million of years

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3

general formula of alkanes

CnH2n+2

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4

bonding in alkanes

  • single covalent bond known as a sigma bond→ a sigma bond is made from the overlapping of 2s orbital from each bonding atom

  • each carbon atom is joined to 4 other atoms via sigma bond

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5

shapes of alkanes

four bonding pairs of electrons around the carbon atom→ tetrahedral shape, 109.5o

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6

what helps alkanes not be rigid

the sigma bond acts as a point of rotation

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7

forces in alkanes

larger the molecule→ greater surface contact→ more London forces→ more energy to overcome→ higher boiling point

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8

how does bp vary in alkanes?

length of chain branching

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9

how does length of chain branching affect bp in alkanes?

there is less surface are contact between the molecules so fewer London forces= lower energy to overcome so low bp

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10

fractional distillation

process of separating crude oil into groups of hydrocarbons with similar number of carbon atoms

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11

crude oil

mixture of hydrocarbons

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12

process of fractional distillation

crude oil is vaporized at 400o → the vaporized hydrocarbons enter the column and rise upwards(temperature gradient = hot at bottom and cool at top)→ different length hydrocarbons will condense at different levels due to the difference in boiling points

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13

where will larger hydrocarbons condense at in fractional distillation and why?

condense at the bottom due to a higher boiling pint

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14

reactivity of alkanes

alkanes are unreactive

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15

why are alkanes unreactive?

the c-c bond (sigma bond) and the c-h bonds in alkanes are very strong and have a high bond enthalpy

c-c bonds are non-polar so the electronegativity difference between C and H is very small

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16

combustion of alkanes

will react with plentiful supply of oxygen to form carbon dioxide and water

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17

incomplete combustion of alkanes

occurs when alkanes are burnt in a limited supply of oxygen, the products vary either being carbon, carbon monoxide or carbon dioxide

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18

why is carbon monoxide dangerous to humans?

binds to hemoglobin at the same area as oxygen but approx 210 times tighter

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19

bond fission

during reactions bonds break and electrons are redistributed

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20

two types of fission

  • heterolytic

  • hemolytic

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21

heterolytic fission

when the bond is broken, both electrons go to one atom, this creates a + and - charged ion

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22

homolytic fission

when the bond is broken, one electron goes to each atoms from the bond, this creates two atoms with an unpaired electron each

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23

what conditions are needed for free radical substitution?

under extreme conditions such as UV light

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24

free radical

an atom with one unpaired electron is called a free radical

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25

photochemical reaction

a reaction bought about by light

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26

what type of reaction is free radical substitution

photochemical

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27

what are the three steps of free radical substitution

  1. initiation

  2. propagation

  3. termination

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28

initiation in terms of radicals

tarts with no radical but ends with a radical

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29

propagation in terms of free radicals

starts with a free radical and ends with one

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30

termination in terms of free radicals

starts with a free radical but does not end with one

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31

do the three steps of free radical substitution for the reaction between methane and chlorine

initiation

  • Cl2 → 2Cl-

propagation

  • CH4 + Cl* → HCl + CH3 *

  • CH3 + Cl2 → CH3Cl + Cl*

termination

  • 2Cl* → Cl2

  • 2CH3* → C2H6

  • CH3*+ Cl* → CH3Cl

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32

limitations of free radical substitution

there are further substitutions creating a mixture of products

substitutions occur at different positions along the carbon chain

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