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A Level Chemistry > Alkenes > Flashcards

Alkenes Flashcards

1
Q

Alkenes

A

A homologous series of hydrocarbon which contains a double bond

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2
Q

What type of hydrocarbon are alkenes?

A

Unsaturated because they contain a double bond thus don’t have the maximum number of hydrogen atoms bonded to it

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3
Q

Alkenes functional group

A

C=C

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4
Q

General formula of alkenes

A

C2H2n

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5
Q

Naming alkenes

A

Number of carbon atoms in longest chain
- number(s) of the carbon that the double bond is on
-ene (if 2 double bonds is diene etc)

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6
Q

How are double bonds formed?

A

When a sigma covalent bond forms
So a pi covalent bond can form

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7
Q

How are sigma bonds formed in carbon?

A

When the 2s orbitals in 2 carbon atoms which each contain a single electron overlap in a straight line

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8
Q

Where is the electron density in the sigma bond?

A

Between the 2 nuclei due to the overlap of orbitals side on in a straight line, on the molecular axis

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9
Q

Why is a sigma bond formed when 2 orbitals overlap?

A

Because the 2 electrons In an orbital are shared and give the highest possible electron density between 2 atoms
Thus the nucleus is attracted electrostatically to this

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10
Q

How do pi bonds form in a carbon atom?

A

After a sigma bond is formed, the 2p orbitals in each carbon atom, which only has 1 electron, overlaps side ways

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11
Q

Where is the electron density in a pi bond?

A

Above the molecular axis and below the molecular axis
Therefore spread out above and below the nuclei

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12
Q

Electrostatic attraction in a pi bond

A

Because the electron density is spread out above and below the nucleus, it is weaker than in sigma bonds where the electron density is in between

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13
Q

Why are sigma bonds stronger than pi bonds?

A

Because in sigma bonds the high electron density is between 2 nuclei thus stronger electrostatic attraction
Whereas in pi bonds the electron density is spread above and below the nucleus

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14
Q

Double bond strength vs single bond strength

A

Even though pi bonds are weaker than sigma bonds, having combined strength of both means they are stronger than single bonds

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15
Q

What reagent are alkenes considered?

A

Nucleophiles because they contain a double bond where there is high electron density thus an area of negative charge

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16
Q

What reagents do alkenes usually react with?

A

Electrophiles (electron-pair acceptors) which usually are attracted to high electron density areas

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17
Q

Reaction of alkenes + hydrogen halogens or hydrogen halides

A

Electrophilic addition because an electrophile is being added to the Alkene without substituting any atoms to make a larger molecule

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18
Q

Alkene + hydrogen reaction name

A

Hydrogenation

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19
Q

What does Alkene + hydrogen form

A

An alkane by adding 2 hydrogen and breaking the double bond

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20
Q

Conditions for hydrogenation of alkenes

A

Nickel catalyst
150°c

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21
Q

What happens to the melting point after hydrogenation of Alkene?

A

You remove the double bond by adding hydrogen to a carbon
Which raises the melting point
Therefore alkane is a solid at room temperature

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22
Q

Halogen + Alkene forms?

A

Dihalogenoalkane
So opening up the double bond to attach 2 halogen atoms = DI

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23
Q

How are halogen atoms added to the Alkene when undergoing electrophilic addition?

A

Adds across the double bond when it breaks
So ensure they are added to different carbons and not the same

24
Q

Step 1 of electrophilic addition of Alkene and halogen

A

The high electron density (negative) area around double bond of Alkene repels electrons in the halogen molecule. Induces a dipole across halogen molecule where closest halogen atom is positively charged

25
Q

Step 2 of electrophilic addition of Alkene with halogen

A

There is heterolytic fission of the halogen molecule
Where the 2 electrons in this bond are given to furthest halogen atom
And so the double bond in Alkene breaks because the 2 electrons in pi bond form bond with closest halogen atom

26
Q

Arrow mechanisms in the heterolytic fission of halogen molecule

A

An arrow from the bond between halogens to the furthest halogen atom to show how 2 electrons in the bond go there
Arrow from double bond in Alkene to closest halogen to show how these electrons are shared thus bonded

27
Q

Step 3 of electrophilic addition of halogen with Alkene

A

We are left with a halogen atom which has 2 electrons from the heterolytic fission thus has lone pair of electrons
And also a positively charged carbocation (the carbon on Alkene which 1sthalogen not bonded to)

28
Q

How are charges shown in the 2nd halogen atom and carbocation in electrophilic addition

A

Lone pair of electrons with a negative symbol on the 2nd halogen atom
A + sign on the empty carbocation that isn’t d+

29
Q

Step 4 of electrophilic addition of halogen atom to Alkene

A

The lone pair of electrons on the halogen atom is attracted to carbocation so they form a bond
Thus the double bond is broken and a dihalogenoalkane has been formed

30
Q

Test for alkenes

A

Shake Alkene with bromine water (brown)
Which will form dibromoalkane which is colourless via electrophilic addition
This bromine water tests for C=C bonds

31
Q

Colour change in Alkene + bromine water test

A

Orange ——> colourless

32
Q

Formation of alcohols by reacting water vapour and alkenes?

A

Steam hydration
Forms alcohols

33
Q

Conditions for steam hydration of alkenes

A

300°c
60atm-70atm
Phosphoric (V) acid catalyst
H3PO4

34
Q

Formation of diols from reacting alkenes?

A

By oxidising alkenes using acidified potassium manganite (VII)

35
Q

What is acidified potassium manganite (VII)?

A

The oxidising agent used to form diols from alkenes
Has symbol H+/MnO₄⁻

36
Q

How do you write the oxidation of alkenes by acidified potassium manganite (VII)?

A

The Alkene
Then an arrow with H+/MnO₄⁻ over it
Then forms a diol

37
Q

What colour is H+/MnO₄⁻?

A

Purple

38
Q

What colour does the solution with Alkene reacted with H+/MnO₄⁻ form (colour of diol)

A

Purple to
COLOURLESS

39
Q

Reaction of hydrogen halides and alkenes forms? And why

A

Forms halogenoalkanes
Because the double bond in Alkene breaks and a halogen atom and hydrogen atom is added across the double bond

40
Q

But what is important about the reaction between hydrogen halides and alkenes?

A

There are 2 possible products:
Minor
Major
If the Alkene is unsymmetrical eg double bond not in middle of carbon chain

41
Q

What determines if the product formed in hydrogen halide + Alkene is major or minor?

A

How stable the carbocation formed is
Because the carbocation formed (carbon the hydrogen DOESNT BOND TO) can be stable or not

42
Q

Types of carbocations formed in order of least stable

A

Primary carbocation
Secondary carbocation
Tertiary carbocation

43
Q

What determines the type of carbocation thus stability?

A

How many alkyl groups each carbocation is bonded to

44
Q

Primary carbocation

A

Bonded to 1 alkyl group
Least stable

45
Q

Secondary carbocation

A

Bonded to 2 alkyl groups
Medium stability

46
Q

Tertiary carbocation

A

Bonded to 3 alkyl groups
Most stable

47
Q

Why does the carbocation being bonded to the most alkyl groups make it most stable?

A

Because the alkyl groups can feed electrons to positively charged carbocation

48
Q

Step 1 of reaction between alkenes and hydrogen halides

A

High electron density area in alkene’s double bond attracts the slightly positive charged hydrogen (because hydrogen halides are polar molecules)

49
Q

Step 2 of reaction between alkenes and hydrogen halides

A

Heterolytic fission of hydrogen halide where 2 electrons in bond are given to halogen and hydrogen joins to a carbon because the double bond breaks up

50
Q

How is the addition of hydrogen from hydrogen halide to an Alkene shown in mechanism?

A

Arrow from bond in hydrogen halide to the halogen molecule to show heterolytic fission
Arrow from double bond to d+ hydrogen to show how pi bond electrons form a bond with H d+

51
Q

Which carbon will hydrogen bond to in major product?

A

The hydrogen will bond to the carbon with most hydrogen bonded to it so the carbocation formed will be the one with most alkyl groups attached to it = stable

52
Q

Which carbon will hydrogen bond to in the minor product?

A

The carbon with the least hydrogen bonded to it already so has more alkyl groups
Thus forms carbocation with least number of alkyl groups to it = less stable

53
Q

Step 3 of reaction between hydrogen halide and Alkene

A

We now have a halogen atom with a lone pair of electrons from the heterolytic fission
And a carbocation formed
These are attracted to each other thus bond to form a halogenoalkane

54
Q

Which carbon does the halogen bond to in the major product?

A

The carbon with the most alkyl groups attached to it because this one is the carbocation
Thus the more stable carbocation

55
Q

Which carbon does the halogen bond to in the minor product?

A

The carbon with the least alkyl groups attached to it because this is a less stable carbocation

56
Q

Markowikoff’s rule

A

The major product from the addition of a hydrogen halide to an unsymmetrical Alkene is the one where hydrogen bonds to the carbon with the most hydrogen already bonded to it

A Level Chemistry (59 decks)

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