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chemistry > Alkenes > Flashcards

Alkenes Flashcards

1
Q

What are alkenes

A

Alkanes are unsaturated hydrocarbons and contain at least one c=c bond

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2
Q

What’s the ending when more than one double bond is present

A

Diene , triene

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3
Q

What do we add to ene when we use diene

A

We add ‘a’ so it would be buta -1,2 diene

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4
Q

What is the shape of an alkene molecule

A

Three regions of electron density surround the central carbon atom . Pairs of electrons repel each other as far as possible to minimise repulsions
This gives a trigonal planar shape with approx 120bond angle
Ethene is a flat planar molecule

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5
Q

Why is it approx 120

A

Because the double bond has a higher electron density and repulsion is greater

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6
Q

How do sigma bonds form

A

Overlap of s orbitals which are spherical

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7
Q

How do pi bonds form

A

P orbitals are dumbbell shaped and can overlap in two ways , either end on forming sigma bonds or sideways overlap forming pi bonds

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8
Q

What is the double bond made up of

A

A sigma bond and a pi bond

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9
Q

Where is the sigma bond formed

A

Directly between the two carbon atoms

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10
Q

Where is the pi bond formed

A

Above and below the plane of the bonding carbon atoms

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11
Q

Draw the formation of a pi bond

A
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12
Q

Do double bonds rotate

A

No because the pi bond fixes the carbon atoms in position , at either end of the double bond this prevents any rotation

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13
Q

How many electrons are used for each bond in carbon

A

3 electrons used for 3x sigma bonds and 1 used for pi bond

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14
Q

What do the bond enthalpies show about double bonds and single bonds

A

Double bonds are stronger than single bonds however a pi bond is weaker than a sigma bond . When an alkene reacts the pi bond breaks and the sigma bond remains intact . As electron density is spread over the top and bottom the pi bond has a lower bond enthalpy .

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15
Q

What are the two types of isomerism

A

E/Z and optical isomerism

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16
Q

What is the definition for stereoisomerism

A

Stereoisomers are compounds with the same structural formula but with a different arrangement or atoms in space

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17
Q

What is E/Z isomerism

A

A type of stereoisomerism in which different groups attached to each carbon of a carbon =carbon double bond may be arranged differently in space because of the restricted rotation of the c=c double bond

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18
Q

What criteria is needed for an E/Z isomerism

A
  • must have a carbon carbon double bond (because of the restricted rotation)
  • each carbon in the double bond must be attached to two different groups
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19
Q

Which is the Z isomer and which is the E isomer

A

Z - groups / atoms are on the same side of the double bond
En- groups /atoms are on opposite sides across the double bond

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20
Q

What is cis /trans

A

Cis /trans nomenclature Can be applied to any such compounds in which two of the substituent groups (one on each carbon atom of the c=c group ) are the same .

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21
Q

The cis isomer is the ….

A

Same on both sides

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22
Q

The trans isomer is ….

A

Different on sides

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23
Q

What is cahn - Ingold prolog nomenclature

A

If the groups with higher priority ( higher atomic number ) are on the same side it is a Z isomer
If the groups with higher priority are on diferent sides it is an E isomer

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24
Q

the double bond of an alkene is an area of ……………

A

high electron density

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25
Q

the negative charge of the double bond may be attacked by electron- deficient species , which will accept ……

A

a pair of electrons

26
Q

what is an electrophille

A

an electron pair acceptor

27
Q

alkenes undergo addition reactions when attacked by electrophiles . this is called …………………………..

A

electrophillic addition

28
Q

what is the definition for an electrophile

A

an electrophile is an atom or group of atoms attracted to an electron - rich centre or atom, where it accepts a pair of electrons to form a new covalent bond

29
Q

explain electrophillic addition

A

1) when a non-polar molecule eg br2 approaches the alkene , the pi electrons in the carbon-carbon double bond interact with the electrons in the covalent bond. this interaction causes polarisation of the bond producing an induced dipole
2) the electron pair in the pi bond is atttracted to the delta positive end of the molecule, causing the double bond to break by heterolytic fission . a bond forms from one carbon atom from the double bond, and an atom from the approaching molecule
3) the bond in the approaching molecule breaks by heterolytic fission , with the electron pair going to the delta negative end of the molecule , forming a negative ion and a carbocation
4) the lone pair of electrons on the negative ion reacts with the positively charged carbon in the carbocation to form the addition product of the reaction

30
Q

what is the definition for a carbocation

A

an organic ion in which a carbon atom has a positive charge

31
Q

what are the examiners tips when drawing electrophillic mechanisms

A
  • make sure arrow comes from middle of double bond to delta positive molecule
  • make sure arrow comes from middle of bond to delta negative molecule .
    -make sure arrow comes from lone pair to carbocation
32
Q

what electrophillic addition reactions do alkenes participate in

A
  • hydrogenation which is the reaction with hydrogen in the prescence of a nickel catalyst.
  • halogenation - reaction with halogens
    -reaction with hydrogen halides
    -hydration-reaction with steam in the prescence of an acid catalyst
33
Q

what are the conditions for the hydrogenation of alkenes

A

nickel catalyst and temperature of 423K

34
Q

what is halogenation of alkenes

A

the reaction between alkenes and halogens

35
Q

what is the test for unsaturation of the carbon chain

A

alkenes decolourise bromine water(orange to clear )
which shows the prescence of a c=c bond. if we add bromine water to a saturated compound there will be no addition reaction. additionally the more unsaturated the compound , the greater the volume of bromine water that can be decolourised

36
Q

what are the conditions for alkenes with hydrogen halides

A

at roon temperature , mix gases together or bubble the gaseous halide through liquid alkene.

37
Q

what happens when you have an unsymmetrical alkene and hydrogen halide

A

you make 2 different products ( as the hydrogen and halide can add onto differetn places on the chain

38
Q

whats the conditions for the hydration of alkenes to form alcohols

A

320 c and high pressure in the prescence of an acid cataylst eg H3PO4 phosphoric acid

39
Q

what happens when an unsymmetrical alkene is reacted with steam

A

2 possible products

40
Q

why are unequal amounts of the products formed

A

due to the relative stabilities of the carbocation intermediates

41
Q

what is markownikoffs rule

A

when a hydrogen halide reacts with an unsymmetrical alkene , the hydrogen of the halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and the smaller number of carbon atoms

42
Q

how do you know which alkene is the major product

A

the most stable carbocation immediate can be found by seeing where the most and least carbons and hydrogens are eg the halide will go where there is most carbon atoms and the hydrogen where there is least carbons and most hydrogens

43
Q

what happens to the stability of the carbocations as the number of alkyl groups on the positively charged carbon increases

A

increases
primary , secondary , tertiary ect

44
Q

What is an addition polymer

A

A giant structure comprised of a very long molecular chain with high molecular mass , formed by repeated addition reactions of many unsaturated alkene molecules

45
Q

What is a monomer

A

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

46
Q

What is addition polymerisation

A

The process in which unsaturated alkene molecules add on to a growing polymer chain one at a time , by repeated addition reactions , to form a very long saturated chain

47
Q

What would ethene become after polymerisation

A

Poly(ethene)

48
Q

How can different polymers be made

A

Polymers with specific properties can be made by varying the monomer and the conditions /catalysts used .

49
Q

What are the general conditions for making a polymer

A

High temperature and pressure in the presence of a catalyst

50
Q

What is a repeat unit

A

A specific arrangement of atoms that occurs in the structure over and over again . They are included in brackets , outside of which Is the symbol n

51
Q

What ways can waste polymers be processed

A

-Separation and recycling
- organic feedstock
-combustion

52
Q

What is separation and recycling

A

Where the waste is sorted into each different type of polymer and then each type can be recycled by melting and remoulding

53
Q

What does feedstock do

A

Can be used as a feedstock for cracking allowing for the new production of plastics and other chemicals

54
Q

What happens with combustion

A

Can be incinerated to produce energy for electricity .

55
Q

What’s a disadvantage of combustion of waste polymers

A

Formation of toxic , acidic waste products eg HCL , however this can be neutralised to neutralise acidic fumes

56
Q

What polymers have chemists been developing

A

Biodegradable polymers. soluble polymers , compostable polymers and photodegradable polymers

57
Q

What’s an advantage of combustion of waste polymers

A

Energy used to produce electricity

58
Q

What’s a disadvantage of combustion of waste polymers

A

Products of combustion cause acid rain , formation of co2 causes global warming

59
Q

Describe how HCl gas is removed from the waste gases produced

A

HCl gas is passed through alkali

60
Q

why is a plasticisiser added to PVC

A

it makes it more flexible

chemistry (18 decks)

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