Topic 6: Organic Chemistry I Flashcards
what is meant by ‘homologous series’?
A family of compounds w/ the same functional group. They differ by CH2 from the next member, with similar chemical and a trend in physical properties
what is meant by ‘functional group’?
an atom/group of atoms in a molecule responsible for su chemical reactions, replacing H in an organic compound
What are isomers?
Compounds w the same molecular formula but different structures.
What is the difference between position, functional group and structural isomers?
Position isomers have the functional group of the molecule in a different position of the carbon chain, eg propan-1-ol and propan-2-ol. They have similar chem but diff physical properties.
Functional group isomers have the same molecular formula but the molecules have a diff functional groups chemical and physical properties, eg hexene and cyclohexane.
Structural isomers have the same molecular formula but a different structural arrangement of atoms. They can be straight or branched chains, eg butane vs 2-methylpropane.
What is stereoisomerism?
Stereoisomers have the same structural formula but have a different spatial arrangement of atoms and bonds.
E/Z is a type of stereoisomerism which occurs due to limited rotation around a C=C.
The limited rotation means that groups attached to the C=C can either be on the same side or on opp sides.
Showing the structure with atoms in a straight line doesn’t show stereoisomerism.
How do you assign priority to E/Z isomers?
In molecules that display E/Z isomerism, the atom or group on each side of the double bond with the higher Mr is given the higher priority. These groups are used to determine if the molecule is the E or Z isomer. If the two atoms attached to the C=C are the same, you need to find the first point of difference after that and then assign priority.
How is E/Z isomerism different to cis trans isomerism?
Stereoisomers can be named cis instead of Z for when the groups are on the same side and trans instead of E for when they are different sides. Cis and trans can ONLY be used when there are hydrogen atoms to compare the other groups to.
What happens to the boiling temp of the alkanes if the chain is straight or branched?
longer chain= more e and more points of contact between molecules so more London forces. This increases bpts Branching lowers bpt due to less points of contact between molecules.
Describe and explain combustion of alkanes, with any potential harmful effects.
Complete combustion releases CO2 and H2O.
In incomplete combustion they release CO and C, which is released as soot. This blackens buildings and causes breathing problems.
Oxides of nitrogen and sulphur are produced as a byproduct of alkane combustion with carbon particulates from unburnt fuel. In clouds these oxides react with water to form acid rain.
These gaseous products can be removed from systems using a catalytic converter. A rhodium catalyst converts harmful products into more stable ones like CO2 or H2O.
Describe fractional distillation of crude oil.
We obtain fuels and alkanes from crude oil w fractional distillation. Crude oil is put into the bottom of the column.
The mixture is vaporised and fed into the fractioning column. The column is hot at the bottom, cool at the top. Vapours rise, cool and condense. Products are siphoned off for diff uses.
Products with short C chains have lower bpts so they rise higher up the column and are collected at the top. Gases are used for cooking, petrol for cars, kerosene is used for plane fuel, Diesel is used for train/lorry fuel, fuel oil is used for ships, Bitumen surfaces roofs/roads. Top fractions are lighter, shorter chains, easier to ignite.
Describe cracking. What is the difference between thermal and catalytic cracking?
Cracking splits up long chain alkanes into smaller useful ones, producing an alkane + alkene.
Thermal cracking produces a high proportion of alkanes and alkenes. 1200K and 7000kPa are used to crack the carbon chains.
Catalytic cracking produces aromatic compounds w carbon rings. Around 720K used with normal Pa, but a zeolite catalyst from Al, Si and O2 is also used to compensate for the less harsh conditions.
Describe the process of reforming
The conversion of straight chain hydrocarbons into branched and cyclic chains.
This is necessary as straight chain hydrocarbons don’t burn as well as branched or cyclic ones.
To convert, they’re heated with a platinum catalyst.
During the reaction H is formed as a byproduct which can be used to make ammonia, so it is useful.
Describe and explain the types of fission.
Bond fission is the different ways in which the chemical bond between 2 atoms can be broken.
Heterolytic fission is when a cv bond breaks and one atom takes both electrons. 2 ions, one + and one - are formed.
Homolytic fission is when a cv bond breaks and each atom takes an electron. Two radicals formed.
A radical is an atom or group of atoms with an unpaired electron.
Show the mechanism of how to form chloromethane (halogenation substitution)
Cl2 + CH4 —-(UV LIGHT)—->
Initiation: Cl2—–> 2Cl°
Propagation 1: Cl° + CH4 —–> °CH3 + HCl
Propagation 2: °CH3 + Cl2 —–> °Cl + CH3Cl
Termination 1: °CH3 +CH3 —–> C2H6
Termination 2: Cl° + Cl° ——-> Cl2
Termination 3: °CH3 + Cl —–> CH3Cl
In the products, the total number of free radicals decreases. ° symbol means a free radical
Why are chain reactions are not a good method for preparing halogenoalkanes?
If an alkane is more than 2 Cs in length, then any H atoms would be substituted, leading to a mix of different isomers. The mixture of products is hard to separate, so it is not a good method.
Some chloromethane molecules formed during the free radical substitution between CH4 and Cl will undergo further substitution to form dichloromethane.
Further substitution can occur till ALL H’s are substituted. These extra chloroalkanes are impurities that must be removed.
The amount of these molecules can be decreased by reducing the proportion of chlorine in the reaction mixture.
Why do we need alternative fuels and what are biofuels?
There is pollution from alkane combustion.
There is depletion of natural resources.
Alkane combustion releases CO2 which contributes to global warming.
Biofuels: Fuels obtained from living matter that has recently died. Bioalcohols are fuels made from plant matter using enzymes or bacteria
Compare renewable versus non-renewable energy
Renewable resources can be continuously replaced.
Non-renewable are not being replenished except over geological timescales.
Why aren’t fossil fuels considered carbon neutral?
Fossil fuels did absorb C02 during their lifetime, but they’re not carbon neutral because the C02 they absorbed was millions of years ago, when the amount was much higher.
Even biofuels must be harvested, transported and processed which all requires energy.
Give disadvantages of fermentation of ethanol
In fermentation, hay an upper limit to the conc of ethanol in the solution.
The ethanol must be separated from a lot of water, which needs energy.
Making ethanol from bacteria and plant waste instead has a much higher upper limit for the conc of ethanol in the solution.
What questions are asked to compare biofuels?
How much land is needed to obtain the fuel?
How much/quickly can a crop grow?
How much energy is needed in manufacture and transport?
How carbon neutral is it?
What are the advantages and disadvantages of plastics/polymers made from alkenes?
They are chemically unreactive so useful for making pipes, packaging.
Easily moulded into attractive shapes.
Lightweight, can easily be coloured
Disadvantages: Low mpts.
When they burn they release toxic gases.
It has C-Hal bonds which bacteria cannot break so they’re non-biodegradable
What happens to waste polymers?
Many polymers are non-biodegradable and indefinitely stay in a landfill.
Energy released from incineration can generate electricity but they’re often toxic emissions.
To recycle, waste needs to be sorted, so reusing is the easiest option.
Describe Alkenes
Alkenes/cycloalkenes are unsaturated hydrocarbons with at least one C=C. Not found naturally in crude oil or natural gas.
Part of a homologous series with the general formula CnH2n. CycloalkAnes are saturated and follow this same general formula.
The C=C in alkenes makes them reactive. During su reactions, the C=C opens up to form single bonds w other atoms.
The bond angle around C=C is 120° due to the overlap of P orbitals. The shape around each carbon is trigonal planar
What is special about the C=C?
The C=C is an area of high electron density making it susceptible to attack from electrophiles (species attracted to electron dense areas). It consists of a normal covalent sigma bond and a pi bond
How do you test for alkenes?
Alkenes decolourise bromine water to change colour from orange/brown to colourless.
This is because the C=C bond can open up to accept bromine atoms, and thus become saturated.
Describe and explain the boiling point of alkenes, and how they combust.
As chain length increases bpt increases due to more e and therefore more Ldn forces that need more energy to break.
Slightly lower bpt than alkanes bc of fewer electrons, and the chain is more kinked.
They burn with a smokey flame due to higher C:H ratio, so it’s not used as fuels
Describe alkene solubility and density
Insoluble in water – but soluble in each other.
All liquid alkenes are less dense than water and will therefore be the upper organic layer in chemical reactions.
The density of the alkenes increases with increasing chain length.
Draw the mechanisms for: electrophilic addition of hydrogen bromide to ethene electrophilic addition of hydrogen bromide to propene
In electrophilic addition of hydrogen bromide to propene, 2-bromopropane forms more often than 1-bromopropane.
This is bc the H joins to the carbon atoms bonded to the most hydrogens.
The Br atom joins to the C atom joined to the most carbons.
Draw the mechanism for Ethene + Bromine
What are primary, secondary and tertiary halogenoalkanes?
The C atom directly bonded to the halogen has a max of 1 other C atom directly bonded to it.
Secondary: The C atom directly bonded to the halogen has 2 other C atoms directly bonded to it.
Tertiary: The C atom directly bonded to the halogen has 3 other C atoms directly bonded to it. Eg 2chloromethylpropane
How do you make an alcohol from a haloalkane? Draw the mechanism for it, using bromoethane and aqueous potassium hydroxide as an example.
Heating a haloalkane with aq potassium hydroxide under reflux makes an alcohol. The attacking nucleophile is the OH- ion. E.g. 1-chloropropane converted to propan-1-ol. Bromoethane is converted to ethanol.
The bond breaking in this mechanism is Heterolytic fission:
Draw the mechanism of the reaction between chloroethane and ammonia heated in a sealed tube.
This reaction forms a PRIMARY AMINE, which forms ethyl amine. There are 2 steps:
What is the difference between primary, secondary and tertiary Alcohols?
P:The carbon atom directly bonded to the OH has a maximum of one other carbon atom directly bonded to it S:The carbon atom directly bonded to the OH has two other carbon atom directly bonded to it T: The carbon atom directly bonded to the OH has three other carbon atom directly bonded to it
Describe the dehydration reaction with butan-2-ol
Heating an alcohol with conc phosphoric acid removes the OH group and a H atom. The 2 possible products are but-1-ene and but-2-ene:
CH3CH(OH)CH2CH3 –> CH2=CHCH2CH3 + H20
CH3CH(OH)CH2CH3 –> CH3CH=CHCH3 + H20
But-2-ene exists as a pair of E/Z isomers. So, hay 3 products: but-1-ene and the 2 isomers of but-2-ene.
The formula for phosphoric acid doesn’t appear in the eqn; the water formed in the reaction mixes with the conc phosphoric acid to dilute the acid.
Generally, how does oxidation affect alcohols?
Unlike dehydration, oxidation only affects one carbon atom.
The atoms removed from an alcohol are the hydrogen of the OH group and a H atom from the carbon atom joined to the OH group as shown:
Why can’t tertiary alcohols be oxidised?
Only the primary and secondary structures have a hydrogen atom on the C of the C-OH group, the tertiary structure does not.
Describe oxidation of a primary alcohol with Acidified Potassium Dichromate (K2Cr2O7/H2SO4)
Primary alcohols become oxidised to form aldehydes (the =O is in the end of the C chain). Unlike ketones, aldehydes are more easily oxidised than alcohols. So when a primary alcohol is oxidised the aldehyde formed may be oxidised further (by gaining 02) to carboxylic acids.
This further oxidation occurs if the oxidising agent is in excess, and the ethanal is prevented from evaporating.
During this reaction, K2Cr2O7 is reduced to hydrated Cr3+ ions. A colour change from orange to green happens.
Describe the oxidation of a secondary alcohol.
Add acidified potassium dichromate and heat under reflux. The secondary alcohol becomes oxidised to form ketones (the =O is in the centre of the C chain)
During this reaction, K2Cr2O7 is reduced to hydrated Cr3+ ions. A colour change from orange to green happens
Which practical technique do you use when you want to obtain a Ketone or a carboxylic acid?
When the oxidation is intended to be complete i.e. to obtain a ketone or carboxylic acid, you must heat under reflux. In this apparatus, products of oxidation stay in the reaction mixture bc, if they do boil off, they condense in the vertical condenser and return to the heating flask.
Which practical technique do you use when you want to obtain an aldehyde?
When the oxidation is intended to be incomplete (to obtain an aldehyde and not a carboxylic acid) the apparatus used is distillation with addition.
In this apparatus only the oxidising agent is heated and the alcohol is slowly added to the ox agent.
When the aldehyde is formed it immediately distils off (it has a much lower boiling temp than the alcohol used to make it), and is collected in the receiver.
