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Topic 6: Organic chemistry 1 > Alkenes > Flashcards

Alkenes Flashcards

1
Q

what is the general formula for alkenes ?

A

CnH2n

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2
Q

how are alkenes hydrocarbons ?

A

they contain hydrogen and carbon only

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3
Q

how are alkenes unsaturated ?

A

there is at least 1 double covalent bond - undergo addition reactions

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4
Q

unsaturated compounds

A

one or more double or triple bonds between atoms in their molecules

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5
Q

what makes alkenes fairly reactive ?

A

double bonds having a high electron density

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6
Q

physical properties of alkenes

A
  • melting and boiling temperatures increase as the number of carbon atoms in the molecules increase
  • do not mix or dissolve in water
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7
Q

what does double bonds contain ?

A

sigma and pi bond

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8
Q

what are sigma bonds ?

A

when 2 s orbitals overlap
- align horizontally to give a single covalent bond
- free rotation is possible around single bonds

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9
Q

due to the high electron density between the nuclei ..

A

there is a strong electrostatic attraction between the nuclei and shared pair of electrons - high bond enthalpy and strongest type of covalent bond

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10
Q

what are pi bonds ?

A

the parallel overlap of 2 p orbitals

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11
Q

where is the pi bond found ?

A

molecules with double or triple bonds - rotation is restricted because of these regions of high electron density

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12
Q

what shape are the p orbitals ?

A

dumb-bell shaped and when they merge they form 2 oblong shaped - one on the top and one at the bottom

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13
Q

why are pi bonds weaker than stigma bonds ?

A

electron density is spread out above and below the nuclei
- the electrostatic attraction is weaker so pi bonds have a lower enthalpy

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14
Q

how many electron are shared in the double bond ?

A

4

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15
Q

how is pi bonds open to attack from electrophiles ?

A

pi bonds stick out a little and the whole double bond has a high electron density

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16
Q

electrophile

A

a chemical species with a vacant orbital that can affect a pair of electrons
- electrophile are positive or have a positive dipole

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17
Q

addition

A

a reaction in which two molecules add together to form a single product

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18
Q

nucleophile

A

a chemical species with a pair of electrons that it can use to form a new covalent bond
- nucleophiles are negative or have a high electron density

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19
Q

electrophilic addition

A
  • alkenes are attacked by electrophiles due to their double bond
  • double bond has a high density of electrons and is attacked by electrophiles - the electrophile adds to the molecule
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20
Q

what is an electrophile

A

electron pair acceptor - they are deficient in electrons and are attracted to the double bond

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21
Q

addition of hydrogen

A

reacting hydrogen gas with ethene at 150 degrees Celsius and a nickel catalyst will make ethane
CH3CH2 + H2 -> CH3CH3

  • this reaction is called catalytic hydrogenation - used in the manufacture of margarine from unsaturated vegetable oils
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22
Q

addition of halogens

A

reacts with br2
to produce di halogenoalkanes

  • alkene + halogen - > 1,2 dihaloalkane
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23
Q

addition of hydrogen halides

A

reacts with HBr
to produce halogenoalkanes

  • alkene + hydrogen halide - > halogenoalkane
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24
Q

Bromine water test

A
  • adding bromine water to a alkene causes a colour change from brown - orange to colourless
  • bromine is the electrophile and adds to the alkene forming dibromo alkane
25
Q

addition of steam

A

acid catalysed hydration of an alkene

alkene + steam - > alcohol

26
Q

oxidation of potassium manganate ( VII )

A
  • adding purple acidified potassium manganate (VII) solution to alkene will lead to the solution turning colourless
  • potassium manganite (VII) is an oxidising agent and oxidises the alkene to make a diol
27
Q

electrophilic addition

A

is the reaction mechanism that shows how electrophiles attack the double bond in alkenes
- when the double bond is broken, a carbocation forms
- this is a carbon atom with only three bonds, meaning it has a positive charge

28
Q

electrophilic addition - mechanism

ethene + hydrogen bromide -> bromoethane

A
  • bromine is more electronegative than hydrogen
  • this means that the bromine pulls electron density toward it
  • carbocation formed is now the electrophile is attacked by the nucleophile bromide ion to form the final product
29
Q

heterolytic bond breaking definition

A

when a covalent bond is broken, one atom takes both of the electrons from the bond - ions are formed

30
Q

carbocation definition

A

a reactive species which contains a carbon atom with a positive charge

31
Q

electrophilic addition - selectivity

A
  • reacting hydrogen halides with unsymmetrical alkanes produce 2 different products
32
Q

how is the amount of the 2 products during electrophilic addition determined ?

A

by the stability of the carbocation intermediate
- the more alkyl groups bonded to the carbocation the more stable the intermediate is

33
Q

Markovnikov’s rule

A

the hydrogen atom from HX adds mainly to the carbon with more hydrogen attached
- the two reaction pathways generate a different carbocation

34
Q

In a primary carbocation the positive carbon is bonded..

A

to one carbon atom

35
Q

In a secondary carbocation the positive carbon is bonded..

A

to two carbon atoms

36
Q

In a tertiary carbocation the positive carbon is bonded..

A

to three carbon atoms

37
Q

which carbocation is the most stable ?

A

tertiary
from primary to secondary to tertiary

38
Q

the more alkyl chains,

A

the stronger the inductive effect and therefore the more stable the cation

39
Q

inductive effect

A

the way in which electrons are either pushed toward or pulled by atoms or groups to which it is bonded

40
Q

addition polymers

A

long chain organic molecules made by the addition polymerisation of alkenes

41
Q

monomer

A

is a small molecule that can join with other monomer units to form a large polymer molecule

42
Q

polymer

A

is a large molecule made from a chain of a monomer units bonded together

43
Q

addition polymerisation - mechanism

A

it is a a free radical addition

44
Q

what are the three stages in the polymer life cycle ?

A

birth - synthesis
life - use
death - disposal

45
Q

green chemistry

A
  • low carbon footprint
  • high energy efficiency in manufacturing
  • clean production process
  • use of renewable resources and renewable energy
  • high safety standards
46
Q

birth of polymers

A
  • when synthesising addition polymers, the regent is an alkene
46
Q

what does current research focuses on ? - birth of polymers

A
  • synthesising polymers based on naturally occurring monomers
  • reducing the energy needed for the process to occur
  • making synthesis, more selective
47
Q

life of polymers

A
  • when synthesising a polymer, a chemist must think about the properties of the polymer and how useful it is to be the consumer
48
Q

poly ( ethene )

A

lightweight, cheap and strong for use in plastic bags

49
Q

poly ( propene )

A

strong, durable, chemical and photo resistivity for use in the furniture and containers

50
Q

poly ( ethanol )

A

flexible and water soluble for use in water soluble bags or capsules

51
Q

death of polymers - name the methods to dispose of polymers

A

landfill
recycling
incineration
using as a cracking feedstock

52
Q

landfill

A

positives: cheap and simple
negatives: lack of biodegradation - polymers break down and form tiny particulates which pollute the environment

53
Q

biodegradable polymers

A

most addition polymers are not biodegradable
- addition polymers are effectively alkanes - highly unreactive

54
Q

biodegradable ( condensation ) polymers

A

poly ( esters )
poly ( amides )
poly ( lactic acid ) - uses: disposable cutlery, plates, bags, hygiene products

55
Q

recycling

A

positives: plastics do not get into the environment
negatives: labour intensive - sorting and takes energy to reprocess and mould the polymer

56
Q

incineration

A

positives: plastic do not get into the environment
negatives: pollutants are released from burning the polymer

57
Q

pyrolysis - polymer -> feedstock

A

positives: pollution into the environment and is lower
negatives: very energy intensive

Topic 6: Organic chemistry 1 (7 decks)

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