4.1.2-Alkanes and Alkenes

what is an alkane

saturated hydrocarbon containing C-C and C-H bonds as sigma (σ) bonds

sigma bonding in alkanes

• caused by overlap of orbitals directly between the two atoms

• free rotation around the σ bond

general formula of alkanes

C_nH_2n+2

how many bonds around each carbon?

what is the bond shape around each carbon?

• 4 σ bonds

• tetrahedral geometry → bond angle 109.5° to minimise electron-electron repulsion between bonding pairs

factors affecting boiling point of alkanes

• chain length

• branching

how does chain length affect bp of alkanes

• as chain length increases for straight chain alkenes, boiling point increases

• larger SA of contact=greater London forces=more energy required to overcome the London forces in order to separate molecules

how does branching affect bp of alkanes

• branched chains have lower boiling points than their straight chain counterparts

• Less SA of contact between molecules=less London forces= less energy required to separate molecules

reactivity of alkanes

• unreactive→ C-C and C-H are strong

• bonds are non polar→ small difference in electronegativity

complete combustion of alkanes

• in presence of sufficient oxygen

• alkane + oxygen → carbon dioxide + water

incomplete combustion of alkanes+ risks

• in presence of insufficient oxygen

• alkane + oxygen → carbon monoxide (or carbon particulates) + water

• carbon monoxide is a toxic gas

• oxides of nitrogen and sulfur produced as by-product→ acid rain

• carbon particulates from unburnt fuel can cause respiratory issues

reactions of alkanes with halogens

• don’t react with halogens without UV light

• in presence of UV a chain reaction called free radical substitution can occur→ forms haloalkane and hydrogen halide

three stages of free radical substitution

1. initiation

2. propagation

3. termination

initiation

• free radicals are formed

• UV light required

• makes two highly reactive radicals

propagation

• free radical reacts with the alkane to form more radicals

• step 1→ halide radical reacts with alkane to form alkyl radical and hydrogen halide

• step 2→ alkyl radical reactions with excess halogen to form haloalkane and halide radical

• halide radical must be preserved→ same radical at start and end of propagation

termination

• free radicals destroyed→ radicals always join together to form an unreactive substance

repropagation

• propagation step can continue many times to result in multiple substitutions→ chain reaction

limitations of free radical substitution

• have low atom economy as many products formed in chain reaction

• presence of reactive free radicals makes radical substitution unpredictable→ can be formed at any C in the alkane

• fractional distillation/chromatography required to purify desired product

Bonding in alkenes

• three bonding regions, no lone pairs

• trigonal planar

• 120° bond angle

types of bonding formed in alkenes

• Sigma (σ) bonding

• Pi (π) bonding

sigma bonding in alkenes

• caused by head on overlap of orbitals

• 3 of 4 valence electrons are used to form 3 sigma bonds:

  • 1 electron electron forms σ bond with other carbon atom

  • 2 electrons for σ bond with hydrogens bonded to carbon

Pi Bonding in alkenes

• caused by the sideways overlap of p orbitals

• creates an area of electron density above and below the plane of the carbon atoms 

• pi bond locks carbon atoms in position→ no free rotation

alkenes general formula

C_nH_2n

are pi or sigma bonds weaker? Why?

• pi bonds are weaker

• sideways overlap of orbitals has smaller orbital overlap than head-on overlap of orbitals formed in sigma bonds

• less energy required to break pi bonds→ only pi bond breaks when alkenes react

chemical test for alkenes

• bromine water

• orange/yellow → colourless

criteria for cis/trans isomerism

• molecule must have a C=C double bond

• each carbon atom must be attached to two different groups

• one of the groups must be hydrogen for both carbons

criteria for E/Z isomerism

• molecule must have C=C double bond

• each carbon atom in the C=C bond must be attached two different groups

Cahn-Ingold-Prelog Priority Rules

1. assign priority to each group attached to the first carbon- atom with highest Ar has highest priority

2. Assign priority to each group attached to the second carbon

3. if two highest priority groups are on the same side→ Z isomer. If on opposite side→ E isomer

what are electrophiles

• electron pair acceptors

• electron deficient and are attracted to electron rich regions in other molecules such as double bonds in alkenes

Examples of Electrophiles

• Hydrogen halides

• Halogen molecules

• Hydrogen molecules

how are hydrogen molecules and halogen molecules electrophiles

• high negative charges of alkene and X₂ molecule in close proximity

• electron repulsion between electron rich C=C of alkene and electron cloud surrounding X₂ molecule

• electron cloud in X₂ molecule shifts away from electron rich double bond→ induces dipole in X₂ with δ+ charge on atom closest to alkene

electrophilic addition mechanism (e.g. bromine + but-2-ene)

• curly arrows must come from bond in stage one

• curly arrow must come from between lone pair in stage 2

hydrogenation of alkenes+ required conditions

• hydrogen+alkene→ alkane

• 150C

• nickel catalyst

hydration of alkenes+ required conditions

• steam added across double bond

• steam + alkene→ alcohol

• 300°C

• phosphoric acid catalyst

stability of carbocations

• primary carbocation→ 1 alkyl group bonded to positive carbon atom

• secondary carbocation→ 2 alkyl groups bonded to positive carbon atom

• tertiary carbocation→ 3 alkyl groups bonded to positive carbon atom

• more alkyl groups= more stable

markownikoff’s rule

• more stable carbocation→ major product

• least stable carbocation→ minor product

types of polymers

• biodegradable polymers

• bioplastics

• photodegradable polymers

biodegradable polymers

• type of plastic that can be decomposed by the action of microorganisms and environmental conditions

advantages of biodegradable polymers

• used as carrier bags

• reused many times

• only degrade to carbon dioxide, water and biological compounds

disadvantages of biodegradable polymers

• some degrade to leave small pieces of non-biodegradable addition polymers→ harmful to environment

• may still require use of non-renewable resources to manufacture

bioplastics

type of biodegradable polymer made from renewable resources e.g. plants starch

advantages of bioplastics

• many uses e.g. bin bags, drinks cups, food wraps

• decompose to leave no toxic residue

• uses renewable resources→ conserves use of non-renewables such as petroleum oil

disadvantages of bioplastics

• may create competition for food sources

• don’t have long re-use life

photodegradable polymers

type of plastic that is broken down chemically using light energy

advantages of photodegradable polymers

• degradation process only requires light

disadvantages of photodegradable polymers

• uses non-renewable resources e.g. petroleum oil

• may not be exposed to enough light to degrade if in landfill

• once exposed to light, begins to break down and not possible to stop process

• break down into small particles of plastic rather than breaking down completely

methods of dealing with polymer waste (5)

• landfill

• combustion

• reusing

• recycling

• feedstock recycling

landfill

rubbish put into large holes in ground and compacted

advantages of landfill usage

• methane gas produced→ used for energy generation

• new regulations ensure they don’ cause water pollution or damage soil

disadvantages of landfill usage

• lots of plastic is non-biodegradable→ can become danger to wildlife

• if leak occurred, leachate can cause groundwater pollution

• not all methane gas can be collected→ fire risk

• no control of what toxic waste ends up in landfill site

combustion

plastics burned/ incinerated in power stations to release energy

advantages of combustion

chemical energy stored in plastics can be transferred to heat energy when burned→ used to drive turbines to generate electricity

disadvantages of combustion

• burning plastics e.g. PVC can produce toxic gases e.g. HCl

• produces CO₂ → greenhouse gas

reusing

some plastics can be reused for same function many times e.g. drinks bottles

advantages of reusing

• helps to conserve non renewable resources

• reduces amount of waste going into landfill

• reduces production of greenhouse gases

disadvantages of reusing

• studies show repeated reuse of plastic bottles increases chance that chemicals will leach out of cracks and crevices in containers

recycling

plastics can be melted down and reshaped into new products

advantages of recycling

• helps conserve non-renewable resources

• reduces amount of waste going into landfill

• reduces greenhouse gas production

• reduces energy consumption→ less energy required to recycle plastics than to manufacture from raw material

disadvantages of recycling

• expensive technology required

• not all polymers easily recycled

• heat energy required to melt plastics comes from non-renewable resources

• melting some plastics can cause release of volatile organic compounds

• recycled plastics are often of a lower quality

feedstock recycling

chemical reactions used to break down plastics into small molecules→ used as raw materials to produce new plastics

advantages of feedstock recycling

• able to handle unsorted and unwashed polymers

• helps conserve non-renewable resources

disadvantages of feedstock recycling

• high investment costs involved in the process

what is a polymer

long molecule made of many smaller monomers

addition polymerisation of alkenes

process when alkene monomers are added to form long chains 

LDPE

• contains branched chains:

  • low density

  • low melting point

  • prevents polymer packing tightly together→ reduced London forces

HDPE

• more straight chains:

  • high density

  • high melting point

  • allows polymer to pack tightly together→ increased London forces