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Chemistry > Alkenes > Flashcards

Alkenes Flashcards

1
Q

What do curly arrows represent?

A

The movement of electrons

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2
Q

What does the double bond of alkenes have?

A

A high electron density

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3
Q

What happens due to the high electron density of carbon carbon double bonds in alkenes?

A

Makes the bond more susceptible to attack by electrophiles

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4
Q

What are electrophiles?

A

Electron loving species

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5
Q

What is electrophilic addition?

A

The addition of an electrophile to a double bond

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6
Q

What does electrophilic addition include?

A

Hydrogen (hydrogenation reaction)
Steam
Hydrogen halide
Halogens

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7
Q

Which catalyst does hydrogenation use and what does it produce?

A

A nickel or platinum catalyst and it produces an alkane
Occurs under heat

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8
Q

What catalyst does reaction with steam use and what does it produce?

A

Phosphoric acid catalyst and it produces an alcohol
Occurs under heat

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9
Q

What do electrophilic additions with hydrogen halides and halogens produce and what conditions do they occur under?

A

Halogenoalkanes
Happens at room temperature

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10
Q

What is hydrogenisation extensively used for in industry?

A

To manufacture margarine

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11
Q

How is margarine produced?

A
  • Naturally occurring vegetable oils are unsaturated
  • Reacted with H2 and become C-C
  • Process changes properties of vegetable oils and converts it to solid
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12
Q

What can alkenes be oxidised and acidified by?

A

Potassium Manganate (VII) (KMnO4) - very powerful oxidising agent

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13
Q

What happens when alkenes are shaken with cold, dilute KMnO4)?

A

Pale purple solution turns colourless and product is a diol ( colour change can be used to distinguish between alkenes and alkanes as alkanes don’t change colour)

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14
Q

How can we think of the reaction between an alkene and an oxidising agent?

A

As an oxidation reaction followed by an addition

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15
Q

What are the steps in reacting an alkene with oxidising agent?

A
  • Potassium manganate solution provides an oxygen atom
  • Water in solution provides another oxygen and 2 hydrogen atoms, so addition of 2 OH groups across the double bond
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16
Q

What is heterolytic fission?

A

Breaking a covalent bond so that the more electronegative atom takes both the electrons from the bond to form a negative ion and leave behind a positive ion

17
Q

What is the charge of hydrogen halides?

A

They are polar
e.g. HBr = H is positive, Br is negative

18
Q

Which atom in HBr would be the electrophile?

A

H as it is electron deficient

19
Q

What happens in an addition reaction of an alkene and a hydrogen halide?

A

The H atom acts as an electrophile and accepts a pair of electrons from the C=C in the alkene

20
Q

What happens to the H-Br bond during electrophilic addition?

A

Breaks heterolytically, forming a Br- ion

21
Q

What happens after the H has received the electron pair from the C=C in electrophilic addition?

A

Results in formation of a highly reactive carbocation intermediate which reacts with the Br- (nucleophile), to form halogenoalkanes

22
Q

What is the charge of halogen molecules?

A

Non-polar

23
Q

What happens when a Br molecule gets closer to the double bond of an alkene?

A

The high electron density in the double bond repels the electron pair in the Br-Br away from the closest Br atom

24
Q

What happens in terms of charge on each Br atom as it gets closer to the C=C in an alkene?

A

Br closest becomes an electrophile and Br furthest away gets a negative charge (there is an induced dipole)

25
Q

What happens in the addition reaction of a halogen with an alkene?

A

Closes Br atoms acts as an electrophile and accepts a pair of electrons from C=C (breaks heterolytically, forming Br- ion)

26
Q

What is formed in the addition reaction of a halogen with an alkene?

A

Results in formation of a highly reactive carbocation intermediate which reacts with Br- to form a halogenoalkane

27
Q

What is the general formula for cycloalkenes?

A

CnH2n-2

28
Q

How does the double bond in an alkene affect its name?

A

Wherever the double bond is, you add that in the name
e.g. double bond on carbon 2 in pentene - would be called pent-2-ene

29
Q

What 2 types of bonding are there in the double bond of an alkene?

A

Sigma and pi

30
Q

How does a sigma bond form?

A

One sp2 orbitals from each C overlap to form a single C-C bond

31
Q

Can rotation occur around a sigma bond?

A

Yes

32
Q

How does a pi bond form?

A

By the sideways overlap of 2 p orbitals on each C atom forming a pi bond above and below the plane of molecule

33
Q

Can rotation occur around a pi bond?

A

There is restricted rotation

34
Q

Is the sigma bond or pi bond stronger?

A

Sigma

35
Q

Are pi bonds exposed and if so what does this mean?

A

Yes - they have a high electron density

36
Q

What is caused by the fact that pi bonds have high electron densities?

A

They are vulnerable to attack by electrophiles

Chemistry (10 decks)

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