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Topic 6: Organic chemistry 1 > Alkenes > Flashcards

Alkenes Flashcards

(58 cards)

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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

Study These Flashcards
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
addition of steam
acid catalysed hydration of an alkene alkene + steam - > alcohol
26
oxidation of potassium manganate ( VII )
- 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
electrophilic addition
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
electrophilic addition - mechanism ethene + hydrogen bromide -> bromoethane
- 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
heterolytic bond breaking definition
when a covalent bond is broken, one atom takes both of the electrons from the bond - ions are formed
30
carbocation definition
a reactive species which contains a carbon atom with a positive charge
31
electrophilic addition - selectivity
- reacting hydrogen halides with unsymmetrical alkanes produce 2 different products
32
how is the amount of the 2 products during electrophilic addition determined ?
by the stability of the carbocation intermediate - the more alkyl groups bonded to the carbocation the more stable the intermediate is
33
Markovnikov's rule
the hydrogen atom from HX adds mainly to the carbon with more hydrogen attached - the two reaction pathways generate a different carbocation
34
In a primary carbocation the positive carbon is bonded..
to one carbon atom
35
In a secondary carbocation the positive carbon is bonded..
to two carbon atoms
36
In a tertiary carbocation the positive carbon is bonded..
to three carbon atoms
37
which carbocation is the most stable ?
tertiary from primary to secondary to tertiary
38
the more alkyl chains,
the stronger the inductive effect and therefore the more stable the cation
39
inductive effect
the way in which electrons are either pushed toward or pulled by atoms or groups to which it is bonded
40
addition polymers
long chain organic molecules made by the addition polymerisation of alkenes
41
monomer
is a small molecule that can join with other monomer units to form a large polymer molecule
42
polymer
is a large molecule made from a chain of a monomer units bonded together
43
addition polymerisation - mechanism
it is a a free radical addition
44
what are the three stages in the polymer life cycle ?
birth - synthesis life - use death - disposal
45
green chemistry
- low carbon footprint - high energy efficiency in manufacturing - clean production process - use of renewable resources and renewable energy - high safety standards
46
birth of polymers
- when synthesising addition polymers, the regent is an alkene
46
what does current research focuses on ? - birth of polymers
- synthesising polymers based on naturally occurring monomers - reducing the energy needed for the process to occur - making synthesis, more selective
47
life of polymers
- when synthesising a polymer, a chemist must think about the properties of the polymer and how useful it is to be the consumer
48
poly ( ethene )
lightweight, cheap and strong for use in plastic bags
49
poly ( propene )
strong, durable, chemical and photo resistivity for use in the furniture and containers
50
poly ( ethanol )
flexible and water soluble for use in water soluble bags or capsules
51
death of polymers - name the methods to dispose of polymers
landfill recycling incineration using as a cracking feedstock
52
landfill
positives: cheap and simple negatives: lack of biodegradation - polymers break down and form tiny particulates which pollute the environment
53
biodegradable polymers
most addition polymers are not biodegradable - addition polymers are effectively alkanes - highly unreactive
54
biodegradable ( condensation ) polymers
poly ( esters ) poly ( amides ) poly ( lactic acid ) - uses: disposable cutlery, plates, bags, hygiene products
55
recycling
positives: plastics do not get into the environment negatives: labour intensive - sorting and takes energy to reprocess and mould the polymer
56
incineration
positives: plastic do not get into the environment negatives: pollutants are released from burning the polymer
57
pyrolysis - polymer -> feedstock
positives: pollution into the environment and is lower negatives: very energy intensive

Topic 6: Organic chemistry 1 (7 decks)

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