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CCEA AS Chemistry Units 1, 2 And 3 > (U2) Alkenes > Flashcards

(U2) Alkenes Flashcards

1
Q

What is an unsaturated hydrocarbon?

A

Compound containing at least 1 C=C or C≡C bond

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2
Q

What is a sigma bond? (2)

A
  • A covalent bond
  • formed by the linear overlap of atomic orbitals (2s, 1s and 1p or 2p)
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3
Q

What is a Pi bond? (2)

A
  • A covalent bond
  • formed by the sideways overlap of p orbitals
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4
Q

What is bond length?

A

The distance between the nuclei of two covalently bonded atoms

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5
Q

What is hydrogenation?

A

Addition of a hydrogen molecule across a C=C bond

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6
Q

What is an electrophile

A

An ion or molecule that attacks regions of high electron density

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7
Q

What is a primary carbocation?

A

A carbocation which has 1 carbon atom directly bonded to the positively charged carbon

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8
Q

What is a secondary carbocation?

A

A carbocation which has 2 carbon atoms directly bonded to the positively charged carbon

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9
Q

What is a tertiary carbocation?

A

A carbocation which has 3 carbon atoms directly bonded to the positively charged carbon

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10
Q

What is polymerisation?

A

Joining together of many monomers to form a large polymer molecule

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11
Q

What are monomers?

A

Many small molecules which join together to form a polymer

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12
Q

What is a polymer?

A

A large molecule formed when monomers join together

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13
Q

What is the general formula of alkenes?

A

CnH2n

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14
Q

What are the physical properties of alkenes? (3)

A
  • boiling point increases with length of carbon chain - more VdV
  • alkenes have 2 fewer electrons than their corresponding alkane, therefore alkanes have a marginally higher b.p. - more VdV
  • insoluble in water, but soluble in organic solvents
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15
Q

Comparatively, which bond is stronger; a pi bond or a sigma bond?

A

Sigma bond - as a result has higher bond energy.

(Pi bonds break easily)

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16
Q

What is formed by the hydrogenation of alkenes?

Under what conditions does the reaction occur?

A
  • Alkanes
  • high temp and pressure (150-300ºC and 4 atm)
17
Q

What is the catalyst for the hydrogenation of alkenes?

Outline the process (5) of hydrogenation of ethene

A
  1. Finely divided nickel
    2.
  • ethene adsorbs onto the catalyst
  • the hydrogen molecule also adsorbs and breaks into atoms
  • the hydrogen atom combines with ethene to form an ethyl radical
  • a second hydrogen atom combines with the ethyl radical
  • ethane desorbs from he catalyst
18
Q

What is the test for alkenes?

A

Bromine water turns from brown to colourless

19
Q

Why are alkenes more reactive than alkanes?

(3)

A

The C=C bond possesses a weak, electron rich π bond:

  • π bond has lower dissociation energy than a sigma bond, meaning it is easier to break; alkanes only have strong sigma bonds
  • due to the π bond, there are regions of high electron density above and below the plane (where the π electrons are delocalised)
  • this makes them susceptible to electrophillic attack
20
Q

What type of mechanism is the addition of hydrogen halides to an alkene?

A

Electrophilic addition

21
Q

Outline the process of addition of bromine to ethene (4)

A
  • bromine molecule approaches the ethene and becomes polarised by the -ve charge of the C=C
  • a loose association forms between the ethene and bromine molecules
  • a C-Br bond forms along with a carbocation and a bromine ion
  • the carbocation is unstable and reacts with the bromine ion

Product = 1,2-dibromoethane

22
Q

Outline the process of addition of HBr to ethene (3)

A

(Solutions of HBr = aqueous)

  • H+ ions act as electrophiles and become attack a region of high electron density around the π bond of the C=C
  • a C-H bond forms along with an intermediate carbocation and a bromide ion
  • the carbocation is unstable and reacts with the bromide ion

Product = bromoethane

23
Q

What type of mechanism is the addition of halogens to an alkene?

A

Addition

24
Q

Why is the major product of the addition of dilute HCl to propene 2-bromopropane rather than 1-bromopropane?

A
  • a secondary carbocation is more stable than a primary one
  • groups tend to push electrons toward the carbocation, stabilising it (positive inductive effect)
  • the secondary carbocation has 2 alkyl groups stabilising, where the primary only has 1
25
Q

Describe how to write the process of addition of Iodine to ethene using curly arrow notation

(4)

A
  • arrow from = to ∂+ iodine atom
  • arrow from bond line (-) between iodine atoms to the ∂- iodine atom
  • arrow from :Br- to C+ on the carbocation
  • product

CCEA AS Chemistry Units 1, 2 And 3 (22 decks)

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