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Organic Chemistry AS Level ~☆Sabrina Belfi ♡♡♡ > Chapter 15 : Hydrocarbons > Flashcards

Chapter 15 : Hydrocarbons Flashcards

Definitions , key ideas and formulae

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1
Q

Definition of alkanes

A

Saturated hydrocarbons with the general formula CnH2n+2

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2
Q

Definition of saturated hydrocarbons

A

Compounds of hydrogen and carbon only in which the carbon-carbon bonds are all single covalent bonds, resulting in the maximum number of hydrogen atoms in their molecules.

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3
Q

Definition of enhanced global warming

A

The increase in the average temperatures around the world as a consequence of the huge increase in in the amounts of CO2 and other greenhouse gases produced by human activity.

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4
Q

Definition of free-radical substitution

A

The reaction in which halogen atoms substitute for hydrogen atoms in alkanes. The mechanism involves steps in which reactive free radicals are produced (initiation), regenerated (propagation) and consumed (termination).

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5
Q

Definition of alkenes:

A

Unsaturated hydrocarbons with a carbon-carbon double bond . Their general formula is CnH2n.

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6
Q

Definition of unsaturated hydrocarbons

A

Compounds of hydrogen and carbon only whose molecules contain carbon-carbon double bond ( or triple bond )and therefore do not have the maximum possible number of hydrogen atoms.

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7
Q

Definition of cracking

A

The process in which large , less useful hydrocarbon molecules are broken down into smaller, more useful molecules.

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8
Q

Definition of addition reaction

A

an organic reaction in which two reactant molecules combine to give a single product molecule.

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9
Q

Definition of polymer

A

A long chain molecule made up of many repeating units

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10
Q

Definition of monomer

A

A small, reactive molecule that reacts to make long chain molecules called polymers.

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11
Q

Definition of polymerisation

A

The reaction in which monomers containing carbon-carbon double bonds react together to form long-chain molecules called polymers.

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12
Q

Are alkanes saturated or unsaturated?

A

Saturated.
The carbon atoms (single bonds ) all display sp^3 hybridisation meaning they have the maximum number of of H atoms in their molecules.

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13
Q

Are there 2 fewer or more H atoms in a cycloalkane compared with its equivalent straight-chain alkane

A

2 fewer because the two end carbons are bonded together, forming a closed ring.

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14
Q

Which type of alkane does not follow the general formula?

A

Cycloalkanes

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15
Q

The first step is fractional distillation of the crude oil. The hydrocarbons in each fraction have similar …..

A

Boiling points

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16
Q

The lower the relative molecular mass of the hydrocarbons, the more …

A

Volatile they are

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17
Q

The lower BP hydrocarbons are collected at the bottom /top of the fractionating columns ?

A

Top

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18
Q

Why are alkanes generally unreactive ?

A

Because there is a very small difference in the electronegativity between hydrogen and carbon. The alkane molecules are non- polar , so they cannot be attacked by polar reagents, such as nucleophiles and electrophiles.

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19
Q

Why are alkanes unreactive with nucleophiles and electrophiles?

A

They have no partial positive charge on any of their carbon atoms to attract nucleophiles, neither do they have areas of high electron density to attract electrophiles.

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20
Q

Alkanes only have two types of reactions :

A
  1. Combustion reactions

2. Substitution by halogens in sunlight , which is called free-radical substitution

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21
Q

What 4 reasons are alkanes combust ed for ?

A
  1. To generate electricity in power stations
  2. To heat our homes and cook our food
  3. To provide energy needed in industrial processes
  4. To provide power for ships, aeroplanes, trains, lorries, buses, cars and motorbikes
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22
Q

What is the equation for the complete combustion of an alkane ?

A

Alkane + oxygen ➡ carbon dioxide + water

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23
Q

What is the equation for the incomplete combustion of an alkane ?

A

Alkane + oxygen ➡ carbon monoxide + water

24
Q

Why is carbon monoxide seen as a pollutant?

A
  1. It is a toxic gas that’s bonds with the haemoglobin in your blood. Haemoglobin molecules can no longer bond to oxygen and so annot transport oxygen around your body. CO is odouress, so this adds to the danger.
25
Q

Nitrogen oxides { N2 (g) + O2 (g) ➡2NO (g)} contribute to the problem of…

A

Acid rain

26
Q

Why is enhanced global warming an issue?

A

The increase in temperature could cause melting icecaps; rising sea levels, resulting in the flooding of low -lying areas of land.

27
Q

……..

A

………..

28
Q

Explain the 3 steps in the substitution of an alkane by a halogen in sunlight, using Cl2 and methane

A

Step 1 : initiation

☆We start with a halogen molecule and and get two radicals formed

        UV light 
Cl2       ➡      2Cl•

Step 2 : propagation

☆we start with a molecule (the alkane) plus a free radical ( halogen) and get a molecule and a different free radical.

CH4 + Cl• ➡ •CH3 + HCl

•CH3 + Cl2 ➡ CH3Cl + Cl•

This stage can continue for long ..

Step 3 : termination

☆we start with free-radicals and end up with a single molecule and no free radicals

E.g.

  • CH3 + Cl• ➡ CH3Cl
  • CH3 + •CH3 ➡C2H6
29
Q

What are the 3 types of reactions we learn in AS for Alkenes?

A
  1. Addition
  2. Oxidation
  3. Addition polymerisation
30
Q

What 5 addition reactions of alkenes are there ?

A
  1. Addition of hydrogen ~ with a Ni catalyst
  2. Addition of steam ~ catalyst = concentrated phosphoric acid, H3PO4
  3. Addition of hydrogen halides
  4. Addition of halogens
  5. The mechanism of electrophilic addition to alkenes
31
Q

Definition of cracking

A

The process in with large, less useful hydrocarbon molecules are broken down into smaller, more useful molecules. Using high temperature and a catalyst.

32
Q

When large alkanes are cracked they form ?

A

Smaller alkane molecules and alkene molecules

33
Q

Are alkenes saturated or unsaturated?

Are they more reactive or less reactive than alkanes ?

A

Unsaturated.

More reactive because of their double bond.

34
Q

The addition of hydrogen, H2 (g) to an alkene :

  1. What catalyst
  2. What temperature
  3. what happens
  4. An example
A
  1. Ni catalyst
  2. 140°C
  3. The double bond breaks and an alkane is formed
  4. E.g. CH2=CH2 + H2 ➡ CH3CH3
    ethene ethane
35
Q

Addition of steam, H2O (g) to an alkene:

  1. What catalyst
  2. What temperature
  3. What pressure
  4. What happens
  5. An example
A
  1. Concentrated phosphoric acid
  2. 330°C
  3. 6 MPa
  4. H3PO4
    An alkene + steam ➡ alcohol
  5. H3PO4
    Ethene + steam ➡ ethanol
36
Q

Addition of hydrogen halides, HX (aq) to alkenes :

What happens? An example

What temperature

A

💡The double bond breaks and the HX adds on

At room temperature

Example :

CH2=CH2 + HBr ➡ CH3CH2Br

☆ with longer, asymmetric alkenes there are always 2 possible products depending on where the Halogen atom sits , X

37
Q

What is the colour of chlorine in solution?

A

Pale green

38
Q

What is the colour of bromine in solution ?

A

Orange/ yellow

39
Q

Explain the test for alkenes or the presence of the C=C bond :

A

The compound to be tested must be shaken in bromine water. If it is unsaturated it will decolorise the water from Orange to clear.

Explanation , the bromine has an addition reaction with the alkene breaking the double bond and adding on to form a new molecule that is colourless.

40
Q

What happens in the addition of halogens , X2 (aq) to alkenes ?
Temperature?

A

The double bond breaks and it simply adds on , @ room temperature

41
Q

What makes alkenes open to be attack by electrophiles?

A

Although the molecule is non-polar , there is an area of high electron density around the C=C bond.

42
Q

The mechanism of electrophilic addition to alkenes :

A polar molecule can act as a electrophile e.g. HBr (polar because of the large difference in electronegativity ). In the mechanism of addition the H e.g. acts as an electrophile and accepts a pair of electrons from the…..

A

C=C bond in the alkene

43
Q

Electrophilic addition to alkenes :

How can a non- polar molecule such as Br2 act as an electrophile? 3 points for simplicity

A
  1. As the Br molecule and the alkene (specifically ethene in thus example ) approach each other, the area of high electron density around the C=C bond repels the pair of electrons in the Br-Br bond away from the nearer Br atom. This makes the nearer Br atom slightly positive and the further Br atom slightly negative.
  2. As the new bond between the C and Br atom forms, the Br -Br bond breaks heterolytically. Br+ and Br : -
  3. The Br- Ion formed then attacks the highly reactive carbocation intermediate. ( so one bromine atom bonds to each carbon atom, producing 1,2-bromoethane
44
Q

What two oxidation reactions are there for alkenes ?

A
  1. With Cold dilute acidified manganate (VII) solution

2. with hot concentrated manganate (VII) solution

45
Q

When an alkene is shaken with Cold dilute solution of manganate (VII) :

  1. What is the temperature ?
  2. What is the colour change ?
  3. What is the product formed ?
  4. What can the test be used for ?
A
  1. Room temperature
  2. From pale purple to colourless
  3. The alkene is converted into a diol - a compound with 2 -OH groups
  4. The reaction is used to fund out whether a compound is unsaturated, if it is it will decolorise the water.
46
Q

What can we use the oxidation of an alkene with concentrated manganate (VII) solution for?

A

To determine the position of the double bond in larger alkenes.

To do this we need to identify the products of the oxidation reaction and work backwards to deduce the original alkene.

47
Q

In the oxidation of an alkene with concentrated manganate (VII) solution , what happens to the C=C bond and what 4 products can the diol formed initially be further oxidised into ?

A

The C=C bond breaks completely.

The diol can be further oxidised into :

  1. Carbon dioxide gas
  2. An aldehyde
  3. A carboxylic acid
  4. A ketone
48
Q

In the oxidation of an alkene with concentrated manganate (VII) solution:

The oxidation products depend on the type of group bonded to a Carbon atom in the C=C.

There are 3 types of groups , give the structure and the product formed (CO2 (g) , an aldehyde, a carboxylic acid and a ketone )

A 
1.     H 
            \
               C=C ➡CO2 (g)
            /
         H 
                                        🔔 carbon  dioxide 🕭
  1. R R
    \ \ 👉Further [O]
    C=C ➡ C=O ➡ R - C ⚌O
    / / \
    H H OH
    🔔 An aldehyde 🔔 🔔 a carboxylic acid 🔔
3,  R1                       R1
        \                           \
         C=C      ➡             C=O 
        /                             /
     R2                           R2 
                              🕭 A ketone 🔔
49
Q

We can summarise the oxidation of an alkene with concentrated manganate (VII) solution , in 3 reactions :

Use ethene

A
  1. H2C =CH2 ➡ CO2 + CO2
  2. RHC =CHR ➡ RCHO + RCHO

☆ IF C IS ATTACHED TO 1 H ATOM AND AN AKYL GROUP WE GET OXIDATION TO A CARBOXYLIC ACID
➡ RCOOH + RCOOH

3.R1R2C=CR3R4 ➡ R1R2C=O + R3R4C=O

50
Q

In addition polymerisation of alkenes what bond breaks ?

A

The pi bond in the C=C bond

51
Q

The section of a polymer shown in brackets is …..

A

The repeat unit of the polymer

52
Q

Give examples of 4 poly (alkenes)

A
  1. Poly (ethene)- used to make carrier bags
  2. Poly (propene)
  3. Poly (phenylethene)
  4. Poly (chloroethene ) =PVC
53
Q

Show the polyermisation reaction of ethene in two ways

A 
n C2H4 ➡-[ C2H4]- n 
Ethene       poly (ethene)

Or the displayed formula

54
Q

Addition polymerisation is characterised by 2 things :

A
  1. Monomers which are unsaturated
  2. The polymer bring the product of the reaction (all you do is break the double bond , put brackets around it and a ‘n ‘ following the bracket )
55
Q

Give two reasons why it is difficult to dispose poly (alkene) plastics ?

A
  1. Their non-biodegradability - takes up valuable space
  2. Their harmful combustion products - contributes to global warming , incomplete combustion could mean CO released. If PVC is burnt , acidic hydrogen chloride gas will be given off , as well as dioxins (other toxic compounds)
56
Q

State Markovnikov’s rule about

Major and minor products :

A

The major product is the one that has the halogen atom bonded to a Carbon with the least number of hydrogens. The product with the more stable carbocation is the major (produced in a higher yield ).

☆ in these reactions two products are formed and you compare the stability of their carbocation to deduce the major and the minor product

☆ the Hydrogen will go to the molecule with already the most H bonds

Organic Chemistry AS Level ~☆Sabrina Belfi ♡♡♡ (5 decks)

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