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CHEMISTRY > >>3.3 - Alkenes✔ > Flashcards

>>3.3 - Alkenes✔ Flashcards

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1
Q

What is the general formula for an alkene?

A
  • CₙH₂ₙ
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2
Q

Describe the bonding in alkenes:

A
  • Bonding in alkenes involves a double covalent bond, a centre of high electron density.
  • Alkenes are unsaturated hydrocarbons as they contain a double carbon bond.
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3
Q

What is the first member of the alkenes?

A
  • C₂H₄
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4
Q

Describe the identification of alkenes:

A
  • Add bromine water to a test tube containing the sample of alkene, it will go from orange to colourless if the sample is an alkene and will remain orange if it is an alkane.
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5
Q

What two ways can we make alkenes?

A
  • Steam or catalytic cracking producing random mixture of alkenes.
  • Elimination of halogenoalkanes with no water and the reagent and halogenoalkane dissolved in ethanol.
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6
Q

What isomers can alkenes form?

A
  • Stereoisomers:
    • E/Z isomers (2 different groups on each carbon.)
  • Structural:
    • Chain (branched.)
    • Positional (e.g: but-1-ene, but-2-ene.)
    • Functional (cycloalkanes and alkenes.)
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7
Q

What is a polymer?

A
  • A long chain of repeating monomer units.
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8
Q

What are the typical uses of poly(chloroethene) / PVC?

A
  • Coats, wire insulation, window frames and drainpipes.
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9
Q

How can poly(chloroethene) properties be modified using a plasticiser?

A
  • By adding plasticiser it makes the plastic more flexible, the more you add the more flexible it becomes.
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10
Q

What are the typical uses of poly(phenylethene) /polystyrene?

A
  • Good insulators.
  • Packaging.
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11
Q

Describe and explain the difference in branched polymers and unbranched polymers properties:

A
  • Polymers with few branches are much more compact and the chains pack closer together so they have more VDW forces, they are stronger, harder and have a higher MP.
  • Branches polymer chains cannot pack as well together and have weaker VDW forces between the chains, these polymers are weaker, softer and have a lower MP.
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12
Q

What is an addition reaction?

A
  • A reaction in which two reactants join together to form one product.
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13
Q

What is an electrophile?

A
  • An electron pair acceptor.
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14
Q

What is a primary carbocation, 1°?

A
  • The positively charged carbon atom is directly attached to one other carbon atom.
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15
Q

Reactions of alkenes - Addition reactions:

Give the equation and conditions for hydrogenation of ethene:

A
  • CH₂=CH₂ + H₂ → CH₃CH₃
  • Nickel, 150°C
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16
Q

Reactions of alkenes - Addition reactions:

Give the equation for the test for unsaturation of ethene:

A
  • C₂H₄ + Br₂ → CH₂BrCH₂Br
  • Orange to colourless.
17
Q

Reactions of alkenes - Addition reactions:

Give the equation for the addition of hydrogen halides (Br) to ethene:

A
  • C₂H₄ + HBr → CH₃CH₂Br
18
Q

What is a secondary carbocation, 2°?

A
  • The positively charged carbon atom is directly attached to two other carbon atoms.
19
Q

What is a tertiary carbocation, 3°?

A
  • The positively charged carbon atom is directly attached to three other carbon atoms.
20
Q

What does the R group within a carbocation stand for?

A
  • Stand for alkyl groups, such as methyl or ethyl, they can be all the same or different.
21
Q

Whats the most stable type of carbocation and what’s the least stable, explain why and why higher stability is good:

A
  • Primary (Least.)
  • Secondary.
  • Tertiary (Most.)
  • Methyl and ethyl groups have an electron releasing effect (positive inductive effect) relative to a hydrogen atom.
  • The more stable the cation the longer the time it remains in the reaction mixture leading to a greater chance of reacting with an anion to form a product.
22
Q

What type of products do more stable carbocations form?

A
  • Major products.
23
Q

What type of products do less stable carbocations form?

A
  • Minor products.
24
Q

Why do tertiary carbocations form major products?

A
  • Tertiary carbocations are more stable than secondary or primary carbocations and so becomes the major product relative to others.
25
Q

What intermediate do electrophilic addition reactions go by?

A
  • Electrophilic addition reactions go via an intermediate carbocation.
26
Q

Electrophilic addition:

Draw the mechanism for electrophilic addition of ethene with hydrogen bromide:

A
27
Q

Electrophilic addition:

For symmetrical alkenes how many positional isomers are possible?

(Give two examples of symmetrical alkenes)

A
  • Only 1 positional isomer possible.
  • e.g: but-2-ene, ethene
28
Q

Electrophilic addition:

For unsymmetrical alkenes how many positional isomers are possible?

(Give two examples of unsymmetrical alkenes)

A
  • Two positional isomers are possible.
  • e.g: propene, but-1-ene
29
Q

Electrophilic addition:

What does Markownikoff’s rule state for electrophilic addition of bromine, hydrogen bromide or sulphuric acid with an alkene?

A
  • In most cases (for hydrogen bromide and bromine) bromine will be added to the carbon with the fewest hydrogens attached to it to give the major product.
  • In most cases (for hydrogen bromide or sulphuric acid) the hydrogen will attach to the carbon with the most hydrogen.
30
Q

Electrophilic addition:

Draw the mechanism for electrophilic addition of ethene with bromine and outline the reagent, conditions and how the bromine forms polar ends:

A
  • IN THE SECOND CARBOCATION, THE HYDROGEN SHOULD BE A BROMINE!
  • Bromine water.
  • Room temperature + pressure.
  • The area of high electron density in the C=C repel the electrons in the neutral bromine molecule to form polar ends.
31
Q

Electrophilic addition:

Draw the mechanism for electrophilic addition of propene with sulphuric acid:

A
32
Q

Electrophilic addition:

What can you do with the product from the electrophilic addition reaction of propene and sulphuric acid?

A
  • Add water to it and heat it to form an alcohol and sulphuric acid.
  • Alkyl-hydrogen sulphate + water → alcohol + sulphuric acid
33
Q

How are addition polymers produced?

A
  • Addition polymers are produced from alkenes where the double bond is broken to form a repeating unit.
34
Q

Draw the addition polymerisation of ethene:

A
  • The repeating unit must always be shown with extended bonds through the brackets showing that it bonds to other repeating units on both sides.
35
Q

In terms of addition polymers what’s a monomer?

A
  • Individual alkene molecules which can combine to form a polymer.
36
Q

In detail describe the melting points of addition polymers:

A
  • Polymers have strong covalent bonds between the atoms however, they have weak IMF between the long chains, a polymer will melt when the IMF forces are overcome, the stronger the force the higher the MP, since the chains are often very long, polymers often have relatively high MP + BP.
  • Since chain length can vary and the strength of the force varies. polymers tend to melt gradually over a range of temperatures rather than at a fixed temperature.
37
Q

Describe and explain the difference in branched polymers and unbranched polymers properties:

A
  • Polymers with few branches are much more compact and the chains pack closer together so they have more VDW forces, they are stronger, harder and have a higher MP.
  • Branches polymer chains cannot pack as well together and have weaker VDW forces between the chains, these polymers are weaker, softer and have a lower MP.
38
Q

Besides from melting points, what are the other 2 key properties of addition polymers?

A
  • Addition polymers are non-biodegradable.
  • Addition polymers are un-reactive as they are saturated.
39
Q

Describe the different reaction conditions for producing; branched chain polymers with weak IMF and straight chain polymers with strong IMF:

A
  • High pressures and temperatures produce branched chain polymers with weak IMF.
  • Lower pressures and temperatures produce straight chain polymers with strong IMF.

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