4.1.3 Alkenes Flashcards
(46 cards)
1
Q
what type of hydrocarbons are alkenes
A
- unsaturated
> contain at least 1 carbon to carbon double bond
2
Q
how is a pi bond formed
A
- by the sideways overlap of adjacent p-orbitals above and below the bonding C atoms
3
Q
what is the nature of the double bond in an alkene
A
- contains a sigma + pi bond
> pi bond locks the 2 Carbon atoms in position, preventing them from rotating around the double bond
4
Q
what is the shape + bond angle around a double bond in alkenes
A
- trigonal planar
- 3 regions repel each other as far as possible, so bond angle is 120
5
Q
what is the general formula of an alkene
A
CnH2n
6
Q
why are alkenes more reactive than alkanes
A
- contain pi bond
> pi electrons are more exposed than electrons in sigma bond due to being on the outside
> pi bond readily breaks + alkenes undergo addition reactions relatively quick
7
Q
what are stereoisomers
A
- compounds with the same structural formula but with a different arrangement in space
8
Q
what conditions are needed for E/Z isomerism
A
- a C=C double bond
- diff groups attached to each carbon atom of the double bond
9
Q
how can you use the Cahn-Ingold-Prelog (CIP) rules to identify E/Z stereoisomers
A
- atoms attached to each carbon atom in a double bond are given priority based on their atomic number
- higher priority groups are used
> if on same side of double bond then Z isomer
> if diagonally places across double bond then E isomer
10
Q
what is cis-trans isomerism
A
- a type of E/Z stereoisomerism where two of the groups attached to each carbon atom of the C=C group are the same
11
Q
name the addition reactions of alkenes
A
- hydrogen in presence of nickel catalyst
- halogens
- hydrogen halides
- steam in presence of an acid catalyst
12
Q
what do each of the addition reactions of alkenes involve generally
A
- addition of a small molecule across the double bond causes the pi bond to break and new bonds to form
13
Q
what are the conditions required for hydrogenation of alkenes
A
- hydrogen
- nickel catalyst
- 423 K
14
Q
what happens in the hydrogenation of alkenes
A
- hydrogen added across the double bond forming an alkane
alkene + H2 = alkane
15
Q
what are the conditions required for the halogenation of alkenes
A
- rapid addition
- aqueous halogen
- room temp
16
Q
what happens in the halogenation of alkenes
A
- halogen added across the double bond, causing it to open + halogen added
17
Q
how can you test for the unsaturation of alkenes
A
- bromine water (orange) turns colourless in presence of a C=C bond
> bromine adds across to the double bond
18
Q
what conditions are required for the addition reactions of alkenes with hydrogen halides
A
- gaseous hydrogen halide
- room temp
- if alkene gas, then both gases mixed
- if alkene liquid, hydrogen halide bubble through it
> can also react with concentrated hydrochloric acid or concentrated hydrobromic acid (solutions of hydrogen halides in water)
19
Q
what happens in the addition reactions of alkenes with hydrogen halides
A
- hydrogen + halogen add to the alkene forming a haloalkane
- can form 2 products - major + minor
20
Q
what are the conditions needed for hydration of alkenes
A
- steam (H2O)
- acid catalyst - phosphoric acid (H3PO4)
21
Q
what happens in the hydration of alkenes
A
- steam adds across the double bond
- produces an alcohol
- can form 2 possible products - major + minor
22
Q
alkenes usually take part in addition reactions to form saturated compounds, what is the mechanism for this reaction called
A
- electrophilic addition
23
Q
what is an electrophile
A
- an electron pair acceptor
> atom / group of atoms attracted to electron-rich centre + accepts an electron pair
> usually a positive ion / molecule containing an atom with a partial positive charge
24
Q
why do alkenes attract electrophiles
A
- double bond in alkenes represents region of high electron density because of pi bond
- high electron density of pi electrons attracts electrophiles
25
what is a carbocation
- contains a positively charged carbon atom
26
what type of fission occurs in the addition reactions of alkenes
- heterolytic fission
> one of the bonded atom gains the electron pair
27
how do non-polar molecules (halogens) undergo electrophilic addition with alkenes
- when halogen approaches alkenes, the pi electrons interact with electrons in halogen bond
> interaction causes polarisation of halogen bond with one molecule becoming slightly positive and the other slightly negative
> known as an induced dipole
28
are alkenes soluble in water
- no
> due to being non-polar
29
what does Markownikoff's rule state
- in a hydrogen halide electrophilic addition with alkenes, the hydrogen attaches itself to the carbon atom of the alkenes with the greater number of hydrogen atoms and smaller number of carbon atoms
30
what is the carbocation stability linked to
- the electron-donating ability of alkyl groups
> each alkyl group donates + pushes electrons towards positive charge of carbocation
> the positive charge is spread over the alkyl groups
> more alkyl groups attached to carbocation, the more the charge is spread out, making ion more stable
31
what are polymers
- large molecules made of repeat units of smaller molecules called monomers
> chain of repeating monomers
32
what do alkenes undergo to produce long saturated chains containing no double bonds
- addition polymerisation
33
what is a repeat unit
- the specific arrangement of atoms in the polymer molecule that repeats over and over again
> always written in square brackets
> place an 'n' after brackets to show a large number of repeats
34
what are the different ways to dispose of waste polymers
- recycling
- PV recycling
- using waste polymers as fuel (incineration)
- feedstock recycling
35
what does biodegradable mean
- can be broken down by enzymes + water
36
why are polymers non-biodegrdable
- saturated
- unreactive
- non polar
37
how are waste polymers recycled
- sorted by type
- chopped into flakes, washes, dried, melted
- recycled polymer is cut into pellets + used by manufacturers to make new products
38
how does PVC recycling work
- solvents dissolve PVC
> high grade PVC recovered by precipitation from the solvents + solvent used again
39
why is PVC disposal + recycling hazardous
- due to high chlorin content + range of additives
- dumping PVC in landfill is not sustainable
- when burnt, PVC releases HCL, a corrosive gas + other pollutants like toxic dioxins
40
how can waste polymers be used as fuel
- derived from petroleum / natural gas so high stored energy value
- incinerated to produce heat
> generates steam to drive a turbine producing electricity
41
what is feedstock recycling
- the chemical and thermal processes that can reclaim monomers, gases, or oil from waste polymers
42
how does feedstock recycling of polymers work
- products from feedstock recycling resembles those produced from crude oil in refineries
> materials used as raw materials for production of new polymers
- can also handle unsorted + unwashed polymers
43
how do biodegradable polymers work
- broken down by microorganisms into water, CO2 + biological compounds
- often starch or cellulose based
44
how do photodegradable polymers work
- bonds weakened by absorbing light to start degradation
- often oil based
45
what are some advantages of polymers
- readily available
- cheap to purchase
46
what are some benefits of biodegradable + photodegradable polymers
- protects environments + conserves valuable oil reserves
> produced from starch, cellulose, plant oils + proteins
> offers a renewable + sustainable alternative to oil based products
