Module 4 - [ch12,13] (alkanes alkenes)

Question 1

what are alkanes

Answer 1

crude oil
among the most stable organic compounds
unreactive
used as fuels
saturated hydrocarbons

Question 2

what does the bonding look like in alkanes

Answer 2

saturated hydrocarbons
only single covalent bonds
each carbon makes 4 bonds

Question 3

what is the shape of alkanes

Answer 3

each carbon atom is surrounded by 4 bonded pairs
tetrahedral
109.5

Question 4

What is meant by the term fractional distillation? [1]

Answer 4

separation by (differences in) boiling point

Question 5

why does boiling point of alkanes increase with chain length

Answer 5

as chain length increases, molecules have larger surface area
more surface contact possible between molecules
london forces between molecules are greater so more energy is required to overcome these forces

Question 6

what is the effect of branching on boiling point of alkanes

Answer 6

branched isomers have lower boiling points

Question 7

what are the reasons for the lack of reactivity of alkanes

Answer 7

C - C C - H
sigma bonds are strong
C - C are non-polar
barely any difference in electronegativity of C and H so can also be described as non-polar

Question 8

combustion of alkanes

Answer 8

alkane + oxygen -> carbon dioxide + water

gives out heat

Question 9

incomplete combustion of alkanes

Answer 9

limited supply of oxygen
toxic gases such a carbon monoxide (CO)
soot

Question 10

when do alkanes react with halogens

Answer 10

in the presence of sunlight
UV radiation provides initial energy for reaction to take place
substitution reaction
methane + bromine -> bromomethane

Question 11

mechanism from bromination of alkanes

Answer 11

initiation
Br - Br -> Br. + Br.
homolytic fission

propagation
CH4 + Br. -> CH3. + HBr
CH3. + Br2 -> CH3Br + Br.

termination
Br. + Br. -> Br2
CH3 + CH3 -> C2H6
CH3. + Br. -> CH3Br

Question 12

what are the limitations to radical substitution

Answer 12

further substitution
substitution at different positions on the carbon chain

Question 13

what are alkenes

Answer 13

unsaturated hydrocarbons
contain a C=C double bond

Question 14

how is a pi bond formed

Answer 14

overlap of adjacent p-orbitals below and above the Carbon atoms
can only be made after a sigma bond is formed

Question 15

describe the shape around a double bond

Answer 15

three regions of electron density around each carbon atom
trigonal planar
120

Question 16

what are stereoisomers

what are the two types

Answer 16

they have the same structural formula but different arrangement of the atoms in space

EZ isomerism (only in compounds with C=C)
optical isomerism

Question 17

what are the conditions for EZ isomerism

Answer 17

. C=C double bond
. different groups attached to each carbon atom of the double bond

Question 18

what is cis-trans isomerism

Answer 18

a special case of EZ isomerism
where one of the attached groups on each carbon atom of the double bond must be the same

Question 19

how do you assign priority

Answer 19

examine the atom directly attached to the carbon atoms of the double atom
greater atomic number = higher priority
if there is no difference, continue down chains till point of difference and compare those atoms

Question 20

cahn-ingold

Answer 20

if highest priority groups are on the same side
(Z)
on opposite sides
(E)

Question 21

why are alkenes more reactive than alkanes

Answer 21

the pi-bond
it is outside of the double bond, therefore more exposed
a pi bond readily breaks and alkenes undergo addition reactions easily

Question 22

hydrogenation of alkenes

Answer 22

alkene + H2 -> alkane

Question 23

halogenation of alkenes

Answer 23

alkene + halogen -> something

Question 24

how do you test for an alkene

Answer 24

bromine water tests for unsaturation (the double bond)
orange -> colourless

Question 25

alkene + hydrogen halide

Answer 25

haloalkanes

Question 26

hydration of alkenes

Answer 26

alkene + water -> alcohol

Question 27

what does the double bond in an alkene represent

Answer 27

a region of high electron density because of the presence of the pi-electrons this high electron density attracts electrophiles

Question 28

what is the mechanism for the addition of a hydrogen halide

Answer 28

Hydrogen halide is polar has dipoles positive dipole attracted to double bond (high electron density) double bond break open and H-Br bond also broken by heterolytic fission, electron pair goes to bromine atom bromide ion and carbocation formed carbocation has a positively charged carbon atom which attracts the ion product formed!

Question 29

what makes the mechanism of the electrophile addition of Br2 different to the addition of HBr

Answer 29

Br2 is non-polar when it approaches the double bond pi-electrons interact with the electrons in the Br2 induced dipole

Question 30

What is Markovnikoff's rule

Answer 30

when a hydrogen halide reacts with an unsymmetrical alkene the hydrogen attaches itself to the carbon with the greater number of hydrogen atoms and least carbon atoms

Question 31

carbocation stability

Answer 31

number of 'alkyl' groups attached to the positively charged carbon, more alkyl groups -> more stable (how many carbons are attached to it) primary carbocations are the least stable, tertiary the most

Question 32

why are tertiary carbocations the most stable

Answer 32

more alkyl groups the charge is more spread out making the ion more stable

Question 33

what are polymers

Answer 33

extremely large molecules made up from repeating units called monomers

Question 34

why am i waffling about polymerisation in this topic

Answer 34

alkene molecules undergo addition polymerisation open the double bond square brackets slap an 'n' at the bottom right corner !polymer!

Question 35

uses of polyethene

Answer 35

most commonly used polymers supermarket bags toys shampoo bottles

Question 36

uses of polychloroethene

Answer 36

PVC pipes, flooring, bottles, insulation

Question 37

polytetrafluoroethene (teflon)

Answer 37

coating non-stick pans permeable membrane for clothing

Question 38

polystyrene (polyphenylethene)

Answer 38

packaging material thermally insulating

Question 39

environmental concerns regarding polymers

Answer 39

disposal -> non-biodegradable recycling PVC hazardous due to high chlorine content when burnt releases hydrogen chloride - a corrosive gas feedstock recycling - reclaiming monomers from waste polymers

Question 40

bioplastics

Answer 40

produced from plant starch, cellulose

Question 41

biodegradable polymers

Answer 41

broken down by microorganisms into water, cO2 and biological compounds compostable polymers

Question 42

photodegradable polymers

Answer 42

weakened by absorbing light

Question 43

what are the conditions for halogenation of alkenes

Answer 43

rapid addition reaction room temp addition reaction

Question 44

what are the conditions for hydrogenation of alkenes

Answer 44

nickel catalyst 150 degrees, 423K addition reaction double bond breaks, addition of hydrogen

Question 45

what are the conditions for hydration of alkenes

Answer 45

alkenes react with steam (H2O) presences of phosphoric acid catalyst (H3PO4) addition reaction

Question 46

electrophilic addition alkene + hydrogen halide what are the conditions

Answer 46

room temp addition reaction (bubbles through if the alkene is liquid)

Question 47

are alkane bonds polar

Answer 47

no carbon and hydrogen have similar electronegativites

Question 48

describe the sigma bond in an alkane

Answer 48

covalent bond direct overlap of s-orbitals

Question 49

what type of intermolecular force do alkanes have and why

Answer 49

london forces because bonds are non-polar

Question 50

are alkanes soluble in water why?

Answer 50

insoluble hydrogen bonds in water are stronger than the london forces in the alkane no polar bonds, so no attraction between water and alkane molecules

Question 51

what reactions will alkanes undergo

Answer 51

combustion reaction with halogens

Question 52

what is the colour of the flame on a bunsen burner during complete combustion

Answer 52

blue

Question 53

how are halogenoalkanes formed from alkanes

Answer 53

radical substitution

Question 54

what intermolecular forces do alkenes have and why

Answer 54

london forces non-polar bonds

Question 55

what will major products be formed from

Answer 55

the most stable carbocation tertiary

Question 56

what are 5 ways in which plastic is disposed of

Answer 56

landfill combustion organic feedstock recycling reuse

Question 57

what is organic feedstock

Answer 57

plastics are separated and broken down into small organic molecules

Question 58

Outline two ways that waste polymers are processed usefully, rather than just dumped in landfill sites [2]

Answer 58

combustion for energy generation feedstock for chemicals, or plastics

Question 59

why do branched alkanes have lower boiling points

Answer 59

have fewer surface points of contact between molecules weaker london forces branches mean the molecules can't get close together decreasing intermolecular forces