12 alkenes Flashcards

1
Q

Physical properties of the alkenes

A
  • Alkenes are unsaturated hydrocarbons containing at least 1 C=C.
  • They have the general formula CnH2n.
  • They typically take part in addition reactions (to the C=C).
  • They exhibit E/Z isomers including the cis / trans isomerism.
  • The C=C is the functional group in an alkene and is responsible for is reactions
  • Ethene is one of the most important alkenes. It is obtained from the catalytic cracking of long chain alkanes. Ethene is used to make polymers for the
    plastics industry.
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2
Q

Definition: A pi-bond

A

(π-bond) is the reactive part of the double bond formed above and below the plane of the bonded atoms by the sideways overlap of p-orbitals. It’s an area of high electron density

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3
Q

The C=C double bond

A
  • The π-bond consists of 2 lobes one on each side of the sigma bond, the 2 lobes
    sideways overlap to produce a π-bond.
  • To ensure maximum overlap, ‘intermittent ethane species’ must be a planar
    molecule.
  • The asymmetric shape of the 2 bonds locks the molecule around the double bond.
    This means that there is no free rotation about these bonds.
  • The π-bond changes the shape around the carbon atom to a trigonal planar with a
    bond angle of 120o:
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4
Q

The shape of alkenes

A

Despite having a C=C - it is simply just one area of electron density.

So the explanation of the geometry is as follows:

Alkenes have three bonding ares in the same plane (3 areas of high electron density)
which repel equally to be as far apart as possible - TRIGONAL PLANAR (120o).

The π-bond changes the shape around the carbon atom to a trigonal planar with a
bond angle of 120o:

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5
Q

Cahn-Ingold-Prelog rules

To have E/Z isomerism, a molecule must have:

A

A carbon-carbon double bond as there is no free rotation around a C=C which
means the geometry is maintained. (rigidity occurs because of the π-bond’s
electron density).

Each carbon in the double bond must be attached to two different groups.

The groups are given priority with reference to their atomic number.

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6
Q

Definition: Stereoisomers

A

compounds with the
same structural formula but with a
different arrangement in space

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7
Q

E/Z stereosimerism aslo kown as

A

(geometric)

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8
Q

priority for stereoisomersim is based of

A

atomic number not mass number

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9
Q

Description of the nature of the double bond c=c

A
  • The extra electrons in the C=C makes it more reactive as it is more electron rich (localised
    electron density).
  • The C=C is it a lot stronger than just a C-C, the C=C is not however twice as strong as the C-C
    (due to orbital overlap)
  • The sigma bond (σ) is a lot stronger than the pi bond (π) which means that the pi bond (π) breaks leaving the sigma bond (σ) intact when an alkene reacts. Pi bond has a low bond enthalpy
  • The σ bond is stronger than the π bond the extent of overlap in sigma bonds is greaterthan that in pi bonds.
  • Alkenes are more reactive than the alkanes due to the electron rich C=C.
  • The C=C is the functional group made
    from a sigma, σ bond and a pi, π-bond.
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10
Q

why do alkenes undergo

Addition reactions of alkenes:

A

Alkenes undergo addition reactions as they are unsaturated.

The reactant will add across the C=C forming a saturated product.

Two reactant molecules react to form one product.

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11
Q

x4 name all 4 and the reaction type

the addition reactions of alkenes

A

1 Addition of hydrogen- hydration/reduction
2 Addition of a halogen- halogenation
3 Addition of steam
4 Addition of hydrogen halides

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12
Q

alkene

Addition of hydrogen

A

Each hydrogen joins to each carbon either side of the C=C.
This reaction is also called hydrogenation
It is also an example of a reduction reaction as hydrogen is added.

conditions:
- H2
- Ni catalyst
- 150 .c

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13
Q

alkenes

Addition of a halogen

A

Each halogen joins to each carbon either side of the C=C.
This reaction is also called halogenation, it makes a dihalogeno alkane
It is also a test for unsaturation.

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14
Q

alkene -> alcohol

A
  • The H joins to one of the carbons while the OH joins to the other carbon in the C=C
  • This reaction is also called hydration as water is added to themolecule. This reaction makes an alcohol.
  • conditons: steam , PHOSPHORIC
    ACID CATALYST
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15
Q

alkenes

Addition of hydrogen halides

A

The hydrogen joins to one of the carbons while the halogen joins to the other
carbon in the C=C.

Any hydrogen halide will react, these are gases and are usually bubbled through
the alkene. This reaction makes a haloalkane.

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16
Q

Definition: A carbocation

A

Definition: A carbocation is
an organic ion in which a
carbon atom has a positive
charge.

17
Q

alkenes

draw The electrophilic addition mechanism

cl-cl h-br

A

The C=C double bond is a region of high electron density due to
the π electrons. This means it will attract electrophiles to itself.

As all the reactions are addition reactions, the reactions are
described as electrophilic addition

18
Q

Addition to unsymmetrical alkenes

A
  • forms two products one major one minor
19
Q

Markownikoff’s rule states…

A

‘A rule used to predict the major and minor product when adding a hydrogen halide to an unsymmetrical alkene’

Rule of thumb:

  • Hydrogen adds to the C of the double bond with greatest number of hydrogenatoms already attached.
  • Halide adds to other carbon.
20
Q

draw the major product

Forming the major product:

A

Once the C=C attacks the H with a partial positive charge, causing the heterolytic fission of the
H-Br bond, a negative bromide ion and positive carbocation remain.

The carbocation formed is positive on the middle C as the hydrogen adds to the first Carbon (this
is called a secondary carbocation)

21
Q

draw the minor product

Forming the minor product:

A

The carbocation formed is positive on the terminal C as the hydrogen adds to the second
carbon (this is called a primary carbocation)

22
Q

Carbocation stability from most ro least stable

A

Most stable
– tertiary carbocation (3 alkyl groups attached)

– secondary carbocation (2 alkyl groups attached)

– primary carbocation (0, or 1 alkyl group attached)
Least stable

23
Q

explain the trend in carbocation stability

A

WHY?

Carbocations are stabilised by alkyl groups.

Alkyl groups are considered to be electron donating – so they push electrons towards the
positive charge, thus spreading out the positive charge onto neighbouring carbons.

The more alkyl groups surrounding the positively charged carbon, the greater the spread of
charge – the more stable the carbocation.

24
Q

Definition: A polymer

A

a
long molecular chain made
up from monomer units.

25
Q

Definition: A monomer

A

a
small molecule that
combines with many other
monomers to form a
polymer.

26
Q

Polymerisation

A

is when an alkene undergoes an addition reaction to itself. Using high temp and pressure.

27
Q

addition polymerisation.

A

The majority of alkenes are used in the polymer industry
forming plastics. Polymerisation is when an alkene undergoes
an addition reaction to itself. Using high temp and pressure.

28
Q

non – biodegradable.

A

resistant to most chemicals and bacteria.
This makes them non – biodegradable.

29
Q

Recycling

A

Polymers are chopped, washed, dried and melted into pellets to make
new products

30
Q

Recycling PVC

A

PVC contains a large chlorine content so is hazardous. When burnt it
releases hydrogen chloride a corrosive gas. so unsuitable

It is currently dissolved in a desirable solvent and recovered by
precipitation.

31
Q

Feedstock recycling

A

Reclaim monomers from waste polymers.

Polymers are cracked to produce a mixture of alkanes, alkenes and
aromatic compounds.

These are used further in the chemical industry. Alkenes can be
polymerised again.

32
Q

Bioplastics

A

Made from starch, cellulose, plant oils and proteins – renewable and
sustainable.

33
Q

Biodegradable.

A

Broken down by microorganisms into water, carbon dioxide and other
biological compounds.

Leave no toxic or visible residue.

34
Q

Photodegradable

A
  • These polymers contain bonds that are weakened by the absorption of
    light
35
Q

the benefits to the environment of development
of biodegradable and photodegradable polymers

A

Benefits of reduced dependency on finite
resources and alleviating problems from disposal of
persistent plastic waste.

36
Q
  • E/Z isomerism
A

an example of
stereoisomerism, in terms of restricted
rotation about a double bond and the
requirement for two different groups to
be attached to each carbon atom of the
C=C group

37
Q

cis–trans isomerism

A

(a special case
of E/Z isomerism in which two of the
substituent groups attached to each
carbon atom of the C=C group are the
same)

Use of E as equivalent to trans and Z as equivalent
to cis is only consi

38
Q

define

an electrophile

A

An electrophile is
an electron pair acceptor
(-tends to have a partial
positive charge). An
electrophile is attracted to
an electron rich centre or
atom where it accepts
electrons to form a new
covalent bond.