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topic 6 Flashcards

1
Q

Alkanes

A

General formula - CnH2n+2

Saturated: Every C atom had 4 single covalent bonds around it

Non polar: No distinct dipole moments present. This makes them unreactive and insoluble as strong, covalent non polar bonds make them resistant to attack by other reactive species and polar water

Varied melting and boiling points: Increases with greater Mr, as stronger London forces as more electrons are involved, so more energy is required to separate molecules. Branching generally lowers melting/ boiling points when compared to straight chain isomers

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2
Q

Structural isomers

A

Molecules with same molecular formula but a different structural formula

Cyclical alkanes are not structural isomers of straight chained alkanes

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3
Q

Crude oil

A

It’s unrefined

Mixture of hydrocarbons

Mainly saturated (alkanes)

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4
Q

Cracking

A

The process of breaking up larger, less useful hydrocarbons into smaller, more useful ones.

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5
Q

Thermal cracking

A

High temperatures - 900C

High pressure - 7000Kpa

Free radicals formed by homolytic fission

These free radicals go on to form:

  • mostly alkenes - used to make polymers
  • smaller alkanes - used as fuels
  • Hydrogen gas - useful in industry/ as fuels
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6
Q

Thermal cracking

A

High temperatures - 900C

High pressure - 7000Kpa

Free radicals formed by homolytic fission

These free radicals go on to form:

  • mostly alkenes - used to make polymers
  • smaller alkanes - used as fuels
  • Hydrogen gas - useful in industry/ as fuels
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7
Q

Catalytic cracking

A

Lower temperatures - 500C

Catalyst- silicon dioxide and aluminium oxide (zeolites)

Heterolytic fission (both electrons go to same carbon) forms carbocations

These carbocations go on to form:

  • Mostly smaller alkanes - fuels
  • Some reforming of alkanes

Chain lengths produced are random. Fractional distillation required.

Catalytic cracking is a more greener process as lower temps and less energy needed

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8
Q

Reforming

A

Straight chain alkanes can form

Branched chain alkanes - fuels

Benzenes

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9
Q

Cyclical alkanes (cycloalkanes)

A

Saturated ring structure alkanes

General formula CnH2n - isomeric with straight chain alkenes

Same physical/chemical properties as alkanes

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10
Q

Complete combustion of alkanes

A

Products are carbon dioxide and water

Reaction of alkane with oxygen

Is exothermic

The greater the chain length, the greater the energy released when the products are formed

However the greater the chain length, the more energy needed to react (harder to burn)

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11
Q

Incomplete combustion of alkanes

A

The products form carbon monoxide (instead of dioxide) and water.

carbon monoxide is highly toxic as it binds to red blood cells

More likely in longer chain alkanes

Some pure carbon (soot) can also be produced

Impurities such as sulphur and nitrogen may also be present in the fuel producing sulphur dioxide and nitrogen dioxide respectively

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12
Q

Environmental impact of combustion:

Carbon dioxide

A

Impact: Global warming

Solution : Use of carbon neutral fuel sources, e.g. biofuels

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13
Q

Environmental impact of combustion:

Sulphur dioxide

A

Impact: Acid rain.

Sulphur dioxide further reacts with oxygen producing sulphur trioxide, which reacts with water to produce sulfuric acid (acid rain)

Solution: desulphurisation: using calcium oxide or calcium carbonate

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14
Q

Environmental impact of combustion:

Nitrogen oxide
Carbon monoxide
Unburned alkanes

A

Nitrogen oxide - Acid rain (nitric acid)
Carbon monoxide - Health issues/smog
Unburned alkanes - Global warming

Solution - Catalytic converters
2CO + 2NO ———-> N2 + 2CO2

C8H18 + 25NO ———> 12.5N2 + 8CO2 + 9H2O

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15
Q

Environmental impact of combustion:

Carbon particulates

A

Impact: Smog, health issues such as cancer

Solution: Use fuels that produce fewer particulates, e.g. petrol produces less than diesel

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16
Q

Free radical substitution

A

Substitution of alkane hydrogen atoms with halogen free radicals

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17
Q

Process of free radical substitution:

Initiation

A

Formation of free radicals

Homolytic fission produces free radicals in the presence of UV light in chlorine

Cl2 ———> Cl• + Cl•

• represents an unpaired electron. Highly reactive species

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18
Q

Process of free radical substitution:

Propagation

A

CH4 + Cl• ————> •CH3 (methyl radical) + HCl

•CH3 + Cl2 ———–> CH3Cl (product) + Cl•

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19
Q

Process of free radical substitution:

Termination

A

Free radicals combine

Cl• + Cl• ———-> Cl2

  • CH3 + Cl• ——–> CH3Cl (which is the product)
  • CH3 + •CH3 ———> C2H6 ( alkane twice the size of original produced)
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20
Q

Process of free radical substitution:

Overall reaction

A

CH4 + Cl2 ———-> CH3Cl + HCl

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21
Q

Process of free radical substitution:

Problems

A
  1. Will not occur in the dark (UV needed)
  2. Substitution is random. No control over which hydrogen substituted in larger alkanes
  3. If left to run, multiple substitutions can occur

Multiple products made! Not precise process

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22
Q

Alkenes

A

General formula: CnH2n

Unsaturated: they contain double bonds between carbon atoms so tend to undergo addition reactions

Reactive: the double carbon bond is an area of high electron density so it’s open to electrophilic attack

Can show geometrical isomerism as the carbon to carbon double bond is non rotational

Non polar: No distinct dipole moments present. This makes them insoluble as strong, covalent non polar bonds make them resistant to attack by polar water

Varied melting and boiling points: Increases with greater Mr, as stronger London forces as more electrons are involved, so more energy is required to separate molecules. Branching generally lowers melting/ boiling points when compared to straight chain isomers

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23
Q

The C=C bond

A

Functional group

Area of high electron density, ie very negative

Open to electrophilic attack

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24
Q

Types of C=C bond:

Bond 1

A

Very strong sigma Bond

Formed between overlap of two s orbitals

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25
Types of C=C bond: Bond 2
Weaker Pi Bond Formed between overlap of two p orbitals Highly negative area in middle so are open to electrophilic attack Pi bonds break during reactions They are weaker than sigma bonds as they are above and below the nuclei so have a weaker attraction
26
testing for alkenes: bromine water
Test: add bromine dropwise to the Organic sample at room temperature Observation: positive = Brown to colourless negative = stage Brown electrophilic addition reaction
27
Secondary test for alkenes: Acidified potassium manganate solution
Test: add acidified potassium manganate solution dropwise to the Organic sample at room temperature Observation: positive = purple to colourless negative = stays purple Redox reaction Test may also give a positive result for alcohols and aldehydes
28
Geometrical isomers
Form of stereoisomerism Geometric isomers have groups that occupy different relative positions in space Specific to alkenes as the carbon to carbon double bond is non rotational
29
Criteria molecules must have in order to show geometrical isomerism
1. Must have a C = C bond | 2. Two different groups bonded to each carbon in the c = c Bond
30
How to know if something shows geometrical isomerism
Focus on c = c Summarise groups Focus on heavier groups "priority"
31
Naming geometrical isomers (E/Z): Cann - ingold prelog system
1. look up atomic numbers of all the atoms that are bonded to each carbon in the c = c Bond 2. highest atomic number takes priority 3. If two atomic numbers are the same on one carbon in the c = c Bond, we move to the next atom in the chain and compare 4. Where are the priority groups relative to each other? Ze zame Zide = Z Opposite sides = E
32
Naming geometrical isomers (E/Z): Cis/ tran system
If the two priority groups in the molecule are the same then the cys/tran notation can be used Same = sis opposite = tran
33
What is electrophilic addition
electrophilic addition is when an alkene is converted into a halogenoalkane
34
Electrolphilic addition Draw step 1 and explain
HX has a permanent dipole H (&+) is the electrophile Pair of electrons are retracted from the pi Bond to the H Heterolytic fission of the HX bond occurs
35
Electrophilic addition Draw step 2 and explain
Carbocation is formed X(..-) is now a nucleophile
36
electrophilic addition step 3
Single halogen atom added to the molecule
37
Difference when a halogen is used in electrophilic addition instead of a hydrogen halide
in step one the dipole is induced by the pi Bond
38
markovnikov's rule
One major and one minor product is made during the addition of an asymmetrical alkene Carbocation 3° most stable 2° 1° Least stable
39
Asymmetrical alkene
different number of hydrogen atoms on each carbon in the c = c Bond
40
Hydrogenation
Alkene is turned into an alkane Reagent is hydrogen gas, needs nickel or platinum catalyst and a temperature of 150°C Very important reaction in food industry. Unsaturated vegetable oils are converted by this process into saturated ones
41
Hydration
Alkene is turned into an alcohol Reagent is water Needs a concentrated phosphoric acid catalyst Temperatures above 100° ( steam)
42
Addition polymerisation
Alkenes can be made into longer saturated hydrocarbons Atom economy = 100%
43
method of showing repeating unit
Break double bonds (pi) Extend bonds out Add square brackets and the letter n
44
Method of finding the monomer
Look for the simplest repeating unit Ignore side groups above and below Take away the square brackets and add the double bond back
45
Polymers
polymers are unreactive as they are saturated molecules Intermolecular forces and therefore physical properties do vary depending on site groups
46
Waste polymers: Recycling
Some polymers can be remoulded into new products (thermosoftening) Some must be chipped and reformed into new products (thermosetting)
47
Waste polymers: Incineration
Plastics can be burnt for energy production Halogen containing plastics eg PVC release toxic gases e.g. HCL These must be removed during the process e.g. by neutralization
48
Waste polymers: Feedstock
May be cracked to produce smaller more useful alkanes and alkenes These could be used for fuels or production of other organic chemicals
49
Degradable plastics
Biodegradable: broken down by enzymes Photodegradable: broken down by UV light
50
Halogenoalkanes
General formula CnH2n+1X Saturated: every carbon atom has four single covalent bonds present Polar: permanent dipole present on the C-X bonds as the X (halogen) is highly electronegative Undergo nucleophilic substitution and elimination reactions Not water-soluble: C-X Bond is not polar enough to interact with the waters intermolecular forces Varied melting and boiling points: dipole dipole intermolecular forces present due to the polar C-X bonds. This results in a higher melting and boiling points than the corresponding alkanes. However the melting and boiling points increases with increased Mr as there are stronger induced dipole forces between nonpolar sections
51
Primary secondary and tertiary halogenoalkanes
Can be primary secondary or tertiary 1° Primary: 2 hydrogens bonded to the same carbon atom as the halogen 2° Secondary: 1 hydrogen bonded to the same carbon as the halogen 3° Tertiary: 0 hydrogen-bonded on the same carbon as the halogen
52
Testing for halogenoalkanes
Unable to test for presence of halogen when bonded to a carbon Nucleophilic substitution releases x - (halide ion) which can be tested for 1. Reflux with aqueous sodium hydroxide - releases X- (halide ion) R-X (aq) + NaOH (aq) ---------> R-OH + Na^+ + X^- 2. Add excess nitric acid - neutralise excess OH- (hydroxide ions) HNO3 + OH- ------> NO3- + H2O 3. Add silver nitrate - identifies halide ions (can follow up using conc/dilute ammonia)
53
When testing for halogenoalkanes why do we add excess nitric acid?
The next step is to add silver nitrate test for halide ions If OH- (hydroxide ions) are present, then the silver ions and hydroxide ions will form silver hydroxide which is a brown precipitate. This interferes with the test Must be nitric acid as hydrochloric acid and sulfuric acid form a white precipitate with silver ions
54
What is nucleophilic substitution
Nucleophilic substitution is when a halogen atom is substituted for nucleophilic group in a halogenoalkane
55
Nucleophilic substitution: forming alcohols (with hydroxide ions aka hydrolysis)
Haloalkene is converted into an alcohol * Reagent is aqueous sodium hydroxide + heat * haloalkane and sodium hydroxide must be mixed in ethanol to make them miscible
56
Draw and explain the first step of nucleophilic substitution when forming alcohols
Lone pair on the OH - is attracted to the Delta positive carbon C-x Bond Breaks by heterolytic fission Overall: R-X + NaOH ---------> R-OH + NaX
57
Nucleophilic substitution: forming a nitrile group (with cyanide ion)
Halogenoalkane is converted into a nitrile Reagent is potassium cyanide dissolved in ethanol + heat Carbon chain length increased by addition of Cn
58
Draw and explain the first step of nucleophilic substitution: forming nitrile with cyanide ion
Lone pair on Cn- is attracted to the Delta positive carbon The c-x bond breaks by heteroytic fission Overall: R-X + KCn -------------> R-Cn=N HXn
59
Nucleophilic substitution: forming an amine group with NH3
Haloalkane is converted into an amine Two step mechanism Reagant is excess concentrated ammonia dissolved in ethanol + high pressure Overall: R-X + 2NH3 -------> R-NH2 + NH4X
60
Draw and explain the first step of nucleophilic substitution forming an amine with nh3
Lone pair of electrons on the nh3 is attracted to the Delta positive carbon C-x Bond Breaks by heterolytic fission NH bonds Breaks by heteroytic fission H+ reacts with more NH3 as a base
61
Relative ease of substitution
The ease of substitution depends on the bond enthalpy of the c-x bond The lower the c-x bond enthalpy, the weaker the bond, the more reactive it is, the easier it is to substitute
62
Rates of substitution practical
Halogenoalkane + water + AgNO3 (heat) -----------> alcohol + H+ + x - As substitution occurs with water, halide ions are released and immediately form a precipitate The time taken for precipitate form can be measured to work out the rate of substitution

chemistry (13 decks)

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