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chemistry > topic 6 > Flashcards

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
Q

Types of C=C bond:

Bond 2

A

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
Q

testing for alkenes:

bromine water

A

Test: add bromine dropwise to the Organic sample at room temperature

Observation: positive = Brown to colourless negative = stage Brown

electrophilic addition reaction

27
Q

Secondary test for alkenes:

Acidified potassium manganate solution

A

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
Q

Geometrical isomers

A

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
Q

Criteria molecules must have in order to show geometrical isomerism

A
  1. Must have a C = C bond

2. Two different groups bonded to each carbon in the c = c Bond

30
Q

How to know if something shows geometrical isomerism

A

Focus on c = c

Summarise groups

Focus on heavier groups “priority”

31
Q

Naming geometrical isomers (E/Z):

Cann - ingold prelog system

A
  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
Q

Naming geometrical isomers (E/Z):

Cis/ tran system

A

If the two priority groups in the molecule are the same then the cys/tran notation can be used

Same = sis

opposite = tran

33
Q

What is electrophilic addition

A

electrophilic addition is when an alkene is converted into a halogenoalkane

34
Q

Electrolphilic addition

Draw step 1 and explain

A

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
Q

Electrophilic addition

Draw step 2 and explain

A

Carbocation is formed

X(..-) is now a nucleophile

36
Q

electrophilic addition step 3

A

Single halogen atom added to the molecule

37
Q

Difference when a halogen is used in electrophilic addition instead of a hydrogen halide

A

in step one the dipole is induced by the pi Bond

38
Q

markovnikov’s rule

A

One major and one minor product is made during the addition of an asymmetrical alkene

Carbocation
3° most stable

2°

1° Least stable

39
Q

Asymmetrical alkene

A

different number of hydrogen atoms on each carbon in the c = c Bond

40
Q

Hydrogenation

A

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
Q

Hydration

A

Alkene is turned into an alcohol

Reagent is water

Needs a concentrated phosphoric acid catalyst

Temperatures above 100° ( steam)

42
Q

Addition polymerisation

A

Alkenes can be made into longer saturated hydrocarbons

Atom economy = 100%

43
Q

method of showing repeating unit

A

Break double bonds (pi)

Extend bonds out

Add square brackets and the letter n

44
Q

Method of finding the monomer

A

Look for the simplest repeating unit

Ignore side groups above and below

Take away the square brackets and add the double bond back

45
Q

Polymers

A

polymers are unreactive as they are saturated molecules

Intermolecular forces and therefore physical properties do vary depending on site groups

46
Q

Waste polymers:

Recycling

A

Some polymers can be remoulded into new products (thermosoftening)

Some must be chipped and reformed into new products (thermosetting)

47
Q

Waste polymers:

Incineration

A

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
Q

Waste polymers:

Feedstock

A

May be cracked to produce smaller more useful alkanes and alkenes

These could be used for fuels or production of other organic chemicals

49
Q

Degradable plastics

A

Biodegradable: broken down by enzymes

Photodegradable: broken down by UV light

50
Q

Halogenoalkanes

A

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
Q

Primary secondary and tertiary halogenoalkanes

A

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
Q

Testing for halogenoalkanes

A

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
Q

When testing for halogenoalkanes why do we add excess nitric acid?

A

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
Q

What is nucleophilic substitution

A

Nucleophilic substitution is when a halogen atom is substituted for nucleophilic group in a halogenoalkane

55
Q

Nucleophilic substitution:

forming alcohols (with hydroxide ions aka hydrolysis)

A

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
Q

Draw and explain the first step of nucleophilic substitution when forming alcohols

A

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
Q

Nucleophilic substitution:

forming a nitrile group (with cyanide ion)

A

Halogenoalkane is converted into a nitrile

Reagent is potassium cyanide dissolved in ethanol + heat

Carbon chain length increased by addition of Cn

58
Q

Draw and explain the first step of nucleophilic substitution:

forming nitrile with cyanide ion

A

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
Q

Nucleophilic substitution:

forming an amine group with NH3

A

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
Q

Draw and explain the first step of nucleophilic substitution forming an amine with nh3

A

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
Q

Relative ease of substitution

A

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
Q

Rates of substitution practical

A

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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