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Organic Chemistry > Unit 3: Section 2- Alkanes and Halogenoalkanes > Flashcards

Unit 3: Section 2- Alkanes and Halogenoalkanes Flashcards

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1
Q

What is the general formula of alkanes?

A

CnH2n+2

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2
Q

What is the general formula of cycloalkanes?

A

Cn2n

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3
Q

What one word could you use to describe alkanes?

A

saturated

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4
Q

What does saturated mean?

A

it only contains single bonds

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5
Q

What is petroleum in other words?

A

crude oil

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6
Q

How could you describe a petroleum fraction?

A

a mixture of hydrocarbons with a similar chain length and boiling point range

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7
Q

What does petroleum consist of?

A

a mixture consisting of mainly alkane hydrocarbons

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8
Q

Describe how fractional distillation of crude oil works (4)

A
  1. oil is vapourised to about 350 C
  2. the vapourised crude oil goes into a fractionating column and rises up through the trays. the largest hydrocarbons don’t vaporise at all, because their boiling points are too high- they just run to the bottom and form a gooey residue
  3. as the crude oil vapour goes up the fractionating column, it gets cooler. because the alkane molecules have different chain lengths, they have different boiling points, so each fraction condenses at a different temperature. the fractions are drawn off at different levels in the column
  4. the hydrocarbons with the lowest boiling points don’t condense, they’re drawn off as gases at the top of the column
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9
Q

In a fractionating column, where would you find long chain hydrocarbons?

A

at the bottom of the column

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10
Q

In a fractionating column, where would you find short chain hydrocarbons?

A

at the top of the column

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11
Q

What would be a use of one of the short chain hydrocarbons in fractional distillation?

A

camping/ fuel gas

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12
Q

At what temperature would petrol be formed at in fractional distillation?

A

40 C

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13
Q

What could bitumen be used for?

A

road surfacing

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14
Q

Briefly describe vacuum distillation

A

Low pressures lower the boiling point of long chain hydrocarbons, this allows heavier fractions to be further separated without high temperatures which could break them down

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15
Q

Define cracking

A

the conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds

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16
Q

What is the general reaction formula when breaking down high Mr alkanes?

A

high Mr alkanes –> smaller Mr alkanes + alkenes + (hydrogen)

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17
Q

Why does cracking need to be done at a high temperature?

A

it is a chemical process involving splitting strong covalent bonds

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18
Q

Name three economic reasons for cracking

A
  • shorter chains more in demand
  • make use of excess larger hydrocarbons
  • products of cracking more valuable
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19
Q

What are the two types of cracking?

A

thermal
catalytic

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20
Q

What conditions are needed for thermal cracking?

A

high pressure (7000kPa)
high temperature (up to 1000C)

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21
Q

What does thermal cracking produce?

A

produces mostly alkenes e.g. ethene used for making polymers and ethanol
can sometimes produce hydrogen- used in Haber process

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22
Q

What conditions are needed for catalytic cracking?

A

slight or moderate pressure
high temperature (450 C)
zeolite catalyst

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23
Q

What does catalytic cracking produce?

A

branched and cyclic alkanes, aromatic hydrocarbons

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24
Q

What can the products of catalytic cracking be used for?

A

used for making motor fuels

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25
What are the advantages of catalytic cracking compared to thermal cracking?
cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used catalyst also speeds up reaction, and time=money
26
Why are alkanes useful as fuels?
burning small amounts releases lots of energy
27
What is the downside of burning alkanes and using them as fuels?
they produce lots of pollutants
28
What are the products of complete combustion?
CO2 H2O
29
What two dangerous products can form in incomplete combustion?
CO C
30
Why is CO toxic?
CO molecules bind to the same sites on haemoglobin molecules in red blood cells as oxygen molecules, so oxygen cant be carried around the body
31
Why is C (soot) toxic?
can cause global dimming cause breathing problems can build up in engines
32
What do catalytic converters do?
they remove CO, NO and unburned hydrocarbons from the exhaust gases, turning them into less harmful chemicals, CO2, N2, and H2O
33
How do catalytic converters have a large surface area?
They have a ceramic honeycomb coated with a thin layer of catalyst metals e.g. rhodium
34
What happens when sulphur in fossil fuel is burnt?
sulphur dioxide will dissolve in atmospheric water and can produce acid rain
35
Why is acid rain so harmful?
it can destroy trees and vegetation, as well as corroding buildings and statues
36
Name a substance high in sulphur content
coal
37
How can sulphur dioxide be removed from power station flue gases?
flue gas desulphurisation gases pass through scrubber containing basic calcium oxide, which reacts with the acidic sulphur dioxide in a neutralisation reaction to form calcium sulfite
38
What is formed in flue gas desulphurisation?
calcium sulfite
39
How does nitrogen oxide form in a car engine?
N2 and O2 react due to the high temperatures and pressure in a car engine
40
Why is NO and NO2 so toxic?
NO- can form acidic gas NO2 NO2- toxic and acidic, can form acidic rain
41
Name three greenhouse gases
carbon dioxide methane water vapour
42
Describe the mechanism of the greenhouse effect
UV wavelength passes through the atmosphere to the Earth's surface and heats up Earth's surface. The Earth radiates out infrared long wavelength radiation The C=O bonds in CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere. This energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed
43
What is the main greenhouse gas?
water vapour
44
Why have CO2 levels risen in recent years?
increased burning of fossil fuels
45
Define free radical
a particle with an unpaired electron
46
How do free radicals form?
when a covalent bond splits equally, giving one electron to each atom
47
What type of reaction is halogens reacting with alkanes?
photochemical reaction
48
What do chlorine and methane react to form?
chloromethane
49
What is the overall reaction of chlorine and methane?
CH4 + Cl2 --> CH3Cl + HCl
50
What type of mechanism is the reaction for chlorine and methane?
free radical substitution
51
What are the three steps in free radical subsitution?
1. initiation 2. propagation 3. termination
52
What essential condition is needed for initiation in free radical substitution?
UV light
53
What happens to chlorine in the initiation stage?
the Cl-Cl bond is broken by homolytic fission, so each atom gets one electron from the covalent bond, forming free radicals
54
Why is chlorines bond broken in preference to the others?
it is the weakest bond
55
What happens in the propagation stage?
the chlorine free radicals are very reactive and remove a H from the methane, leaving a methyl free radical the methyl free radical reacts with a Cl2 to produce the main product (CH3Cl) and another Cl free radical
56
What happens in the termination stage?
the free radical form stable molecules
57
What happens if two free radicals join together?
they make a stable molecule, the two unpaired electrons make a covalent bond
58
What are CFcs?
halogenoalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine atoms
59
How are halogenoalkanes named?
chloro- iodo- bromo-
60
What classifies a halogenoalkane as primary, secondary or tertiary?
the number of carbons attached to the C-X functional group
61
What are the two types of reaction that halogenoalkanes can undergo?
substitution elimination
62
What is a nucleophile?
an electron pair donor
63
Give three examples of nucleophiles that can react with halogenoalkanes
OH- CN- NH3
64
What happens in a nucleophilic substitution reaction?
a nucleophile reacts with a polar molecule by kicking out the functional group and taking its place
65
What does the rate of substitution reactions depend upon?
the strength of the bond between the carbon and the halogen
66
Which is the strongest and weakest C-X bond in the halogens?
strongest- C-F weakest- C-I
67
Define hydrolysis
the splitting of a molecule using water
68
How can water react with halogenoalkanes?
water is a poor nucleophile, but it can react slowly with a halogenoalkane in a substitution reaction
69
How can halogenoalkanes react to form alcohols?
react with warm, aqueous hydroxide ions e.g. sodium or potassium hydroxide
70
Why is important that aqueous solutions of hydroxides are used in nucleophilic substitution?
if the solvent is changed to ethanol, then an elimination reaction occurs
71
How can nitriles be formed?
by reacting halogenoalkanes with cyanide ions must be warm usual reagent: KCN dissolved in ethanol
72
What conditions do you need to form amines from halogenoalkanes?
warm a halogenoalkane with excess ethanolic ammonia under pressure
73
Why can the yield of amine decrease after a nucleophillic substitution reaction?
further substitution reactions can happen between the halogenoalkane and the amines formed leading to a lower yield of amine
74
How can you stop the yield of amine decreasing in a substitution reaction?
by using excess ammonia
75
What is an elimination reaction?
the removal of a small molecule (often water) from the organic molecule
76
What is the change in functional group in an elimination reaction with a halogenoalkane and alcoholic hydroxide ions?
the halogenoalkane forms an alkene
77
What are the conditions needed for an elimination reaction?
in ethanol, heat
78
What does under reflux mean?
heating a substance without loosing any of it
79
What type of bond forms in an elimination reaction?
C=C
80
What are the two types of reaction that can occur when a halogenoalkane is reaction with a hydroxide?
nucleophilic substitution elimination
81
What reaction occurs when a halogenoalkane reacts with a hydroxide in aqueous conditions?
nucleophilic substitution
82
What reaction occurs when a halogenoalkane reacts with a hydroxide in anhydrous conditions?
elimination
83
What is a use of chloroalkanes and chlorofluoroalkanes?
solvents
84
Why is the naturally occuring ozone (O3) layer in the upper atmosphere beneficial?
it filters out much of the sun's harmful UV radiation
85
Why is ozone in the lower atmosphere considered a pollutant?
it contributes to the formation of smog
86
Explain how CFCs form a hole in the ozone layer
Chlorine atoms are formed in the upper atmosphere when energy from UV radiation causes Cl-Cl bonds in CFCs to break. The chlorine free radical catalyses the decomposition of ozone due to these reactions because they are regenerated, and O3 becomes 02.

Organic Chemistry (7 decks)

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