module 4

ozone layer being formed and reformed equations

CFCs depleting the ozone layer

The C-Cl bond breaks because it has the lowest bond enthalpy once the UV radiation provides sufficient energy

the chlorine radical formed is a very reactive intermediate. It can react with an ozone molecule, breaking down the ozone into oxygen

propagation step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone. It has been estimated that a single CFC molecule can promote the breakdown of 100,000 molecules of ozone

nitrogen radicals depleting the ozone layer

• nitrogen oxide radicals are formed naturally during lightening strikes and as a result of aircraft travel in the stratosphere

where were CFCs used

refrigerants

air-conditioning units

aerosol propellants

naming haloalkanes

• prefix is added to the name of the longest chain to indicate the identity of the halogen

• when two or more halogens are present they are listed in alphabetical order

• they can be classed as primary, secondary and tertiary

reactivity of the haloalkanes

• halogen atoms are more electronegative that carbon atoms

• the electron pair in the carbon-halogen bond is therefore closer to the halogen atom than the carbon atom. The carbon halogen bond is polar

• the carbon atom has a slightly positive charge and can attract species containing a lone pair of electrons. Species that donate a lone pair of electrons are known as nucleophiles

nulceophile

an atom or group of atoms that is attracted to an electron deficient carbon atom, where it donated a pair of electrons to form a new covalent bond.

nucleophiles include:

-hydroxide ions

-water molecules

-ammonia molecules

hydrolysis of halogen in nucleophilic substitution reaction

• the nucleophile OH-, approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom

• this direction of attack by the OH- ion minimises repulsion between the nucleophile and the halogen atom

• a lone pair of electrons on the hydroxide ion is attracted and donated to the carbon atom

• a new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom

• the carbon halogen bond breaks by heterolytic fission

• the new organic product is an alcohol. A halide ion is also formed

• halogen atoms can be converted to alcohols using aqueous sodium hydroxide

carbon-halogen bond strength

C-F bond is the strongest carbon-halogen bond and the C-I bond is the weakest

• iodoalkanes react faster than bromoalkanes

• bromoalkanes react faster than chloroalkanes

• fluoroalkanes are unreactive as a large quantity of energy is required to break the C-F bond

combustion of alcohols

alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water. The reaction is exothermic and as the number of carbon atoms in the alcohol chain increases, the quantity of heat released per mole increases

oxidation of alcohols

• primary and secondary alcohols can be oxidised by an oxidising agent

• the usual oxidising mixture is a solution of potassium dichromate acidified with dilute sulphuric acid

• if the alcohol is oxidised then the orange solution containing dichromate ions is reduced to a green solution containing chromium ionsoxidatio

oxidation of primary alcohols

primary alcohols can be oxidised to either aldehydes or carboxylic acids. The production of the oxidisation depends on the reaction conditions used because aldehydes are themselves also oxidised to carboxylic acids

preparation of aldehydes

• on gentle heating of primary alcohols with acidified potassium dichromate, an aldehyde is formed

• the ensure that the aldehyde is prepared rather than the carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms

• this prevents any further reactions to the oxidising agent

• the dichromate ions change colour from orange to green

preparation of carboxylic acids

• if a primary alcohol is heated strongly under reflex, with an excess of acidified potassium dichromate, a carboxylic acid is formed

• use an excess of acidified potassium dichromate to ensure that all of the alcohol is oxidised

• heating under reflex ensures that any aldehyde formed initially in the reaction also undergoes oxidation to the carboxylic acid

oxidation of secondary alcohols

• secondary alcohols are oxidised to ketones. It is not possible to further oxide ketones using acidified dichromate ions

• to ensure the reaction goes to completion, the secondary alcohol is heated under reflux with the oxidising mixture

• the dichromate ions change colour from orange to green

oxidation of tertiary alcohols

tertiary alcohols do not undergo oxidation reactions, The acidified potassium dichromate remains orange when added to a tertiary alcohol

dehydration of alcohols

• an alcohol is heated under reflux in the presence of an acid catalyst such as concentrated sulphuric acid or phosphoric acid

• the product of the reaction is an alkene

• dehydration of an alcohol is an example of an elimination reaction

dehydration reaction

any reaction is which is water molecule is removed form the starting material

substitution reaction of alcohols

• when preparing a haloalkane, the alcohol is heated under reflux with sulphuric acid and a sodium halide the hydrogen bromide is formed in site

  NaBr(S) + H2SO4(aq) → NaHSO4(aq) + HBr(aq)

• the HBr formed reacts with the alcohol to produce the haloalkane

naming alcohols

The suffix -ol is added to the stem containing the largest carbon chain. The position of the functional group along the chain is indicated using numbers

physical properties of alcohols

compared to alkanes they are:

• less volatile

• have higher melting points

• greater water solubility

• differences become smaller as the length of the carbon chain increases

reasons of the physical properties of alcohols

• The alkanes have non-polar bonds because the electronegativity of hydrogen and carbon are very similar

• the alkane molecules are therefore non-polar

• the inter molecular forces between non-polar molecules are very weak London forces

• alcohols have a polar O-H bond because of the different in electronegativity of the oxygen and hydrogen atoms

• alcohol molecules are therefore polar

• the intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar O-H group

why is there a difference in volatility of boiling points between alkanes and alcohols

• in the liquid state, intermolecular hydrogen bonds hold the alcohol molecules together

• These bonds must be broken in order to change the liquid alcohol into a gas

• this requires more energy than overcoming the weaker London forces in alkanes, so alcohols have a lower volatility that the alkanes with the same number of carbons

why is there a difference in the solubility of water between alkanes and alcohols

• a compound that can form hydrogen bonds with more is far more water soluble that a compound that cannot

• alkanes are non-polar compounds and can not form hydrogen bonds with water

• alcohols such as methanol and ethanol are completely soluble in water, as hydrogen bonds form between the polar -OH group of the alcohol and the water molecules

• as the hydrocarbon chain length increases in size, the influence of the -OH group becomes relatively smaller, and the solubility of longer-chain alcohols becomes more like that of hydrocarbons - solubility decreases.

classifying alcohols

alcohols can be classed as primary, secondary or tertiary. This classification depends on the number of hydrogen atoms and alkyl groups attached to the carbon atom that contains the alcohol functional group

primary alcohols

in primary alcohols, the -OH group is attached to a carbon atom which is attached to two hydrogen atoms and 1 alkyl group. (methanol is the exception and its still classed as a primary alcohol)

secondary alcohols

the -OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups. Propan-2-ol and pentan-3-ol are both examples of secondary alcohols

tertiary alcohols

the -OH group is attached to a carbon atom that is attached to no hydrogen atoms and three alkyl groups. 2-methylpropan-2-ol is an example

what type of polymerisation do alkenes do

• addition polymerisation

• made up of small alkenes called monomers

why are polymers very useful

They are very unreactive but this makes getting rid of them an issue

different solutions for getting rid of plastics

• buried

• reused

• burned

• creating biodegradable plastics

waste plastics being buried

landfill is an option for dealing with plastics. It is used when:

• its difficult to separate from other waste

• not sufficient in quantities to make separation financially worthwhile

• too difficult to technically recycle

but because the amount of waste we generate is becoming more and more of problem, there’s a need to reduce landfill as much as possible

waste plastics can be reused

many plastics are made from non-renewable oil-fractions, so it makes sense to reuse plastics as much as possible

• some plastics can be recycles by melting and remoulding them

• some plastics can be cracked into monomers, and these can be used as an organic feedstock to make more plastics or other chemicals

burning waste plastics

• if recycling isn’t possible for whatever reason, waste plastics can be burned - and the heat can be used to generate electricity

• This process needs to be carefully controlled to reduce toxic gases for examples, polymers that contain chlorine such as PVP produce HCL when they are burnt which needs to be removed

• waste gases from the combustion are passed through scrubbers which can neutralise gases such as HCl by allowing them to react with a base

biodegradable polymers

• biodegradable polymers decompose quickly in certain conditions because organisms can digest them

• biodegradable polymers can be made from renewable raw materials such as starch or oil fractions, such as the hydrocarbon isoprene, but at the moment they are more expensive than non biodegradable equivalents

• They need certain conditions such as moisture and oxygen to biodegrade

• This means that all of the polymers need to be collected and separated from the non-biodegradable plastics

• There are various potential uses - eg. plastic sheeting used to protect plants from the frost can be made from polyethene with starch grains embedded in it. In the time the starch is broken down by microorganisms and the remaining polyethene crumbles into dust

• scientists have also started developing photodegradable polymers which degrade when exposed to sunlight

electrophilic addition

• alkene double bond opens up an atoms are added to the carbon atoms

• Electrophilic addition reactions happen because the double bond has got plenty of electrons and is easily attacked by eletrophiles

electrophiles

• electron pair acceptors

• usually positively charged ions

• polar molecules where the partially positive area is attracted to the electrons

Producing ethane from ethene

Reacts with hydrogen gas in an addition reaction to produce ethane

It will need a nickel catalyst and a temperature of 150

mechanism for halogens reacting with alkenes

making alcohols from alkenes

• made by steam hydration

• 300 degrees and a pressure of 60-70atm

• phosphoric (V) acid catalyst is needed

• Reaction is reversable

• only produces a low yield of about 5% but you can recycle the unreacted alkene gas making the overall yield much better

alkene reaction with hydrogen halides

• undergo addition reactions to form haloalkenes

two products formed by adding halides to unsymmetrical alkenes

• carbocations with more alkyl groups are more stable because the aklyl groups feed electrons towards the positive charge

• The more stable cation is more likely to form

  least stable → most stable

  primary → secondary → tertiary

Markownikoff’s rule

The major product from addition of a hydrogen halide to an unsymmetrical alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached.

Why can double bonds not rotate

• Carbon atoms in a carbon carbon double bond and the atoms bonded to these carbons all lie in the same plane (they are planar)

• because of the way they are arranged, They are said to be trigonal planar

• The bond angles in the planar units are all 120 degrees

• atoms can’t rotate around them like they can around a single bond because of the pi bonds

• Atoms can still rotate around the single bonds in an alkene

• The restricted rotation causes alkenes to form stereoisomers

what are stereoisomers

They have the same structural formula but a different arrangement in space

Why can alkenes form stereoisomers

Because of the lack of rotation around the double bond when the two double bonded carbon atom have two different groups attached to them then stereoisomerism can take place

Z isomer vs E isomer of but-2-ene

Cahn-Ingold-prelog rules to work out E vs Z isomers

• atom with the highest atomic number has the highest priority

• Highest priority on the same side of the double bond = Z

• opposite sides = E

cis-trans isomers

• if the carbon atoms have at least one group in common, then you can call the isomers cis or trans

• ‘cis’ means the same groups are on the same side of the double bond

• ‘trans’ means the same groups are on different sides of the double bond

unsaturated hydrocarbons

contain at least one carbon carbon double bond

general formula of alkenes

CnH2n

sigma bond

• formed when two S orbitals overlap

• The two S orbitals overlap in a straight line - this gives the highest possible electron density between the nucleic (single covalent bond)

• The high electron density between the nuclei means there is a strong electrostatic attraction between the nuclei and the shared pair of electrons

• They have a high bond enthalpy - strongest type of covalent bonds

Pi bond

• The sideways overlap of two adjacent P orbitals

• Its got two parts to it - one above and one below the molecular axis. This is because the P orbitals which overlap are dumb-bell shaped

• Pi bonds are much weaker than sigma bonds because the electron density is spread out above or below the nuclei. This means that the electrostatic attraction between the nuclei and the shared pair of electrons is weaker, so pi bonds have a relatively low bond enthalpy

Why are alkenes more reactive than alkanes

• alkanes only contains C-C and C-H sigma bonds which have a high bond enthalpy and so are difficult to break. They are also non polar so don’t attract nucleophiles or electrophiles

• Alkenes are more reactive than alkanes because the double bond contains both a sigma and a pi bond

• the double bond contains 4 electrons so it has a high electron density and the pi bond sticks out above and below the rest of the molecule. These two factors mean that the pi bond is likely to be attacked by electrophiles. The low bond enthalpy of the pi bind also contributes to the reactivity of alkanes

reactivity of alkanes

• alkanes do not react with most common reagents

• the sigma bonds are stronger

• and c-c bonds are non polar

• the c and H have very little differences in electronegativity

complete combustion of alkanes

incomplete combustion of alkanes

carbon monoxide

• a colourless, odourless, highly toxic gas

• combines with the haemoglobin which prevents them from carrying oxygen around in the bloodstream

reaction of alkanes with halogens

• react in the presence of UV light

• a substitution reaction

3 stages for the mechanism for the bromination of alkanes

initiation, propagation, termination

What is the mechanism for the bromination of methane an example of

radical substitution

initiation

• the covalent bond in a bromine molecule is broken by homolytic fission

• each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals

• the energy for this bond fission is provided by UV ration

Propagation

• the reaction propagates through two propagation stages, a chain reaction

• In the first propagation step, a bromine molecule reacts with a C-H bond in the methane, forming a methyl radical and a molecule of hydrogen bromine

• in the second propagation stage, each methyl radical reacts with another bromine molecule, forming the organic product bromomethane and another bromine radical

• The new bromine radical then starts this process over again

termination

• two radicals collide, forming a molecule with all electrons paired

• there are a number of possible termination steps

• when two radicals collide and react, both radicals are removed from the reaction mixture stopping the reaction

limits of radical substitution in organic synthesis

further substitution and substitution at different positions along a carbon chain

further substitution

• another bromine bromine radical can collide with a bromoethane molecule, substituting a further hydrogen atom to form dibromoethane CH2Br2

• further substitution can continue until all hydrogen atoms have been substituted

substitution at different points along the carbon chain

• for methane and ethane, there is only one monosubstituted product possible

• if the carbon chain is longer, there a more positions in the carbon chain

• with further substitution, there are even more possibilities

bonding in alkanes

• saturated hydrocarbons

• contains only carbon and hydrogen atoms

• each carbon atom is joined to 4 other atoms by single covalent bonds. These are a type of covalent bond called a sigma bond

sigma bond

• a sigma bond is the result of the overlap of two orbitals, one from each bonding atom

• each overlapping orbital contains one electron, so the sigma bond has two electrons which are shared between the bonding atoms

Shape of alkanes

• each carbon atoms is surrounded by four electron pairs in four sigma p=bonds

• repulsion between these electron pairs results in a 3D tetrahedral arrangement around each carbon atom with the bond angle being approx 109.5

• the sigma bonds act as axis at which the atoms can rotate around freely

variation in boiling point of the alkanes

• boiling point increases as the carbon chain increases

• there are more electrons therefore the London forces get stronger

effect of chain length on boiling point

• London forces act between molecules that are in close surface contact

• as the chain length increases, the molecules have a large area so more surface contact is possible between molecules

• the London forces in between the molecules will be greater so more energy needed to overcome the forces

effect of branching on boiling point

• decreases as branching increases

• fewer points of contact therefore fewer London forces

• the branches get in the way and prevent molecules from getting as close to each other

types of bond fission

homolytic or heterolytic fission

homolytic fission

• each of the bonded atoms takes one of the shared pair of electrons from the bond

• each atom now has a single unpaired electron

• each atom or groups of atoms with an unpaired electron is called a radical

heterolytic fission

when a covalent bond breaks by heterolytic fission, one of the bonded atoms takes both of the electrons from the bond

• the atom that takes both electrons becomes a negative ion

• the atom that does not take the electrons becomes a positive ion

curly arrows

• in a reaction mechanism, curly arrows are used to show the movement of electron pairs when bonds are being broken or made

curly arrows and homolytic fission

addition reactions

• in an addition reaction, two reactants come together to from one product

substitution reaction

In a substitution reaction, an atom or group of atoms is replaced by a different atom or a group of atoms

elimination reaction

an elimination reaction involves the removal of a small molecule from a larger one. In an elimination reaction, one reactant molecule forms two larger products

structural isomerism

compounds with the same molecular formula but different functional group

isomers with the same functional group

In compounds with the same functional group, the functional group can be at different positions along the carbon chain

isomers with different functional groups

• sometimes two molecules with the same molecular formula can have different functional groups

• Eg, aldehydes and ketones with the same number of carbon atoms have the same molecular formula

detecting isomerism by smell

molecular formula

the number of type of atoms of each element present in a compound

empirical formula

the simplest whole number ratio of the atoms of each element present in a compound

general formula

the simplest algebraic formula for any member of a homologous series

displayed formula

Shows the relative positioning of all of the atoms in a molecule and the bonds between them

structural formula

uses the smallest amount of detail necessary to show the arrangement of atoms in the molecule. Branched chains are in brackets

Skeletal formula

A simplified organic formula where:

• all of the carbon and hydrogen labels are removed

• all bonds to hydrogen atoms are removed

• a line represents a single bond

• an intersection of two lines represents a carbon atom

• the end of a line represents a CH3 group

aliphatic

carbon atoms are joined up to each other in unbranched or branched chains or in non-aromatic rings

alicyclic

carbon atoms which are joined to each other in ring structures with or without branches

aromatic

some or all of the carbon atoms are found in a benzene ring

alkanes

containing single carbon carbon bonds

alkenes

containing at least one double carbon carbon bond

alkynes

contains at least one triple carbon carbon bond

names of alkanes 1-10 carbons

How to name alicyclic alkanes

1. identify the longest continuous chain of carbon atoms

2. add the prefix cyclo

how to name alkenes

1. identify the longest continuous chain of carbon atoms

2. identify where the double bond is

3. combine the suffix, stem and position of where the double bond is