Organic Chemistry

Organic chem terminology

organic chemistry = chemistry of carbon compounds

enabled by carbon’s ability to form several strong covalent bonds with itself

a functional group is a certain atom or group of atoms that give the molecule they’re part of certain physical, chemical properties

organic compounds with the same functional group AND general formula but a different number of carbon atoms belong to a homologous series

the start of an organic compound’s name is dictated by the number of carbon atoms it contains. the first 4 four are:

• meth-

• eth-

• prop-

• but-

remember with the anagram My Elephant Prefers Bananas.

there are different formulae you might be asked to present an answer as.

general: composition of any member of a whole homologous series

• alkAnes: C_nH_2n+2

• alkEnes: C_nH_2n

displayed: shows the spatial arrangement of all the atoms and bonds. when you’re drawing these, check that every carbon has 4 lines coming off of it.

molecular: number of each atom in a molecule

structural: presents the structure as letters, showing only double/triple bonds

e.g: butene

general formula	CnH2n
displayed formula
molecular formula	C4H8
structural formula	CH3CH2=CHCH3

Crude Oil

formed over millions of years as biomass was crushed by layers of sand and mud.

high pressure, temperature and lacking oxygen caused crude oil to form

crude oil is a mixture of different hydrocarbons of different sizes. hydrocarbons are compounds that contain ONLY carbon and hydrogen → you must say ‘only’ to get the mark

crude oil alone is not useful, but its fractions are. a fraction is a mixture of different hydrocarbons of a similar carbon chain length (and thus with similar properties)

we can seperate the fractions from crude oil via fractional distillation. this process is often a 6 marker.

1. crude oil is vaporised by heating to 350 degrees C and enters the fractionating column

2. crude oil vapours rise up the fractionating column, which has a negative temperature gradient (temp decreses as vapours move up)

   • the first fractions to condense are those with the longest chains = highest boiling points, e.g. bitumen

     • why? because longer chains have stronger intermolecular forces which take more energy to overcome

   • the last fractions to condense are those with shortest chains = lowest boiling points, e.g. liquid petroleum gas

3. the vapours of a certain fraction condense on a bubble tray at their boiling point, are collected and transported away for use

crude oil fractions are used as fuel + as feedstock for the petrochemical industry:

these are not all the fractions of crude oil, just the ones you need to know for the exam.

other products of the petrochemical industry, that use certain crude oil fractions, are solvents, lubricants, polymers and detergents

Alkanes

crude oil is formed primarily of alkanes

these are saturated hydrocarbons, meaning every possible bond is filled with hydrogen because they only have single bonds between carbon atoms

suffix is -ane

general formula: C_nH_2n+2

e.g: ethane, C₂H₆

properties of alkanes

as you change the length of the carbon chain, the properties of the alkane change

longer alkanes are:

• less flammable (harder to light, burn with a smoky flame)

• are more viscous (harder to pour, flow less easily)

• less volatile (less likely to turn into a gas, because boiling points are higher due to stronger intermolecular forces)

vice versa for shorter alkanes.

reactions of alkanes

alkanes are generally unreactive but they do undergo combustion, can react with halogens in presence of light and can be cracked.

(complete) combustion:

• alkane + oxygen → water + carbon dioxide gas

• carbon and hydrogen are oxidised (because oxygen is gained)

• often asked to balance these equations

• if there isn’t enough oxygen to enable complete combustion of an alkane, unburnt hydrocarbons will be left over, causing environmental issues → see chemistry of the atmosphere notes. this is a problem with larger hydrocarbons e.g. octane (which incidentally is what petrol is mainly comprised of) because they need more oxygen to combust completely.

cracking:

• shorter carbon chain hydrocarbons are in higher demand because they can be used as vehicle fuel

• but crude oil is mostly long chains, which are less useful and of lower demand

• so we crack excess long chain hydrocarbons down into smaller ones to meet demand

• cracking produces a smaller alkane and also an alkene (usually ethene or propene)

• when you’re writing cracking equations, make sure your reactant is an alkane and the products are a smaller alkane + an alkene. also ensure that the number of C and H atoms are the same on each side

• there are 2 ways to crack molecules:

  1. catalytic cracking

     • heating hydrocarbons to 470 to 550 degrees C → vaporises them

     • vapours pass over powdered aluminium oxide catalyst

     • covalent bonds in molecules broken down - thermal decomposition

  2. steam/thermal cracking

     • higher temperatures than catalytic cracking

     • vapourised hydrocarbons mixed with steam and heated more → cracking

     • more alkenes and hydrogen formed due to higher temperature and pressure.

Alkenes

alkenes are unsaturated hydrocarbons, meaning not every bond is filled with a hydrogen atom because they have a double bond between 2 carbon atoms on their chain

suffix is -ene

functional group is C=C

general formula: C_nH_2n

e.g: ethene, C₂H₄

methene does not exist because there’s only 1 carbon atom; no double bond can be formed. so the 4th one you need to know is pentene.

reactions of alkenes

alkenes are very reactive because their C=C bond can open up to enable more bonds to be made with other atoms.

combustion:

• alkenes are more susceptible to incomplete combustion than alkenes because of the higher carbon:hydrogen ratio

• this produces smoky yellow flames

• the possible products of incomplete combustion are CO (carbon monoxide) or C (as soot)

alkenes undergo addition reactions - where the simple molecule the alkene is reacting with is added across the C=C bond, making it into a C-C bond.

when you draw the display formulae for these reactions, you need to make sure you’re adding the new molecules where the C=C bond is being broken; not just anywhere in the molecule.

• where the C=C bond is is denoted by a number in the middle of the name. e.g prop-2-ene has the C=C between the 2nd and 3rd C atoms.

1. with halogens (halogenation)

   • occurs at room temperature

   • reaction with bromine water used as a test for alkene - solution turns orange → colourless if it’s an alkene because the bromine gets added across C=C - no longer free in the solution

     

2. with hydrogen (hydrogenation)

   • nickel catalyst

   • at 150 degrees C

   • forms an alkane

   • e.g. ethEne + hydrogen → ethAne

     

3. with water (hydration)

   • phosphoric acid catalyst

   • pressure of 60 atm

   • temperature of 300 degrees C - so the water is water vapour

   • forms an alcohol

   • reversible reaction

   • after the reaction, unreacted alkene is seperated by condensing the mixture because its boiling point is much lower

   • the left over water and the alcohol are seperated by fractional distillation

   • e.g: ethene + water vapour → ethanol

     

Alcohols

functional group OH

suffix -anol

general formula C_nH_2n+1OH

note that their molecular formula has the OH at the end - e.g. propanol = C₃H₇OH, NOT C₃H₈O. the OH is the functional group, so we don’t combine it with the other Hs.

alcohols are colourless, dissolve in water to form neutral solutions

first 4 are commonly used as fuels for lab equipment because they burn cleanly (no smoke) and without a strong odour. ethanol is used in car fuel

methanol and ethanol are often used as solvents because they can dissolve many substances water cannot (e.g. fats, oils) + the ones water can.

ethanol is the alcohol used in alcoholic drinks

producing ethanol

there are 2 ways to produce ethanol:

1. hydrating ethene. the ethene is gathered from cracking of long chain alkanes. for more detail on reaction see above

2. fermentation.

   1. extract glucose form crops

   2. add yeast to glucose; enzymes in the yeast act as a catalyst

   3. ferment the glucose - 15-35 degrees C, absence of oxygen

      • glucose + enzymes → ethanol + carbon dioxide

   • batch process (stops and has to be started again) because once ethanol concentrated exceeds 15%, the yeast are killed off and have to be replenished

reactions of alcohols

combustion: produce carbon dioxide and water; the alcohol is the fuel

with sodium:

• alcohol + sodium → sodium alkoxide + hydrogen

• fizzing (H gas bubbles) and sodium decreases in size during the reaction

• e.g. ethanol + sodium → sodium ethoxide + hydrogen

  note that there’s no line between the O and the Na because they’re ionically bonded. The ½ in front of H₂ is for balancing

  

with an oxidising agent:

• oxidising agents induce oxidation in an alcohol

• this can occur naturally (e.g. in the case of wine) or artificially (e.g. by using potassium dichromate)

• redox reaction

• alcohol + oxidising agent → carboxylic acid + water

• e.g. ethanol + potassium dichromate → ethanoic acid + water

  

Carboxylic acids

functional group COOH

suffix -anoic

carboxylic acids are weak acids - they only partly ionise in water to produce weakly acidic solutions. this causes them to form aqueous solutions with high pHs of 3-7.

reactions of carboxylic acids

with water:

• soluble in water, produce a weakly acidic solution

with carbonates:

• carboxylic acid + carbonate → salt + water + carbon dioxide

• like another other acid

• e.g. propanoic acid + potassium carbonate → potassium propanoate + water + carbon dioxide

with bases:

• like any other acid produce a salt and water

esterification

when a carboxylic acid reacts with an alcohol, an ester is formed. this is usually done in the presence of a concentrated sulfuric acid catalyst

ester functional group is R₁-COO-R₂ where R1 is the carboxylic acid and R2 the alcohol.

(glucose)are sweet smelling and oily, used in food flavouring or perfume. they’re volatile (vapourise easily)

Polymerisation

creating large molecules of high relative molecular mass from a lot of small molecules (monomers). each monomer is a repeat unit and is connected to adjacent repeat units via covalent bonds.

addition polymerisation

polymerisaiton of molecules with a C=C bond (not ONLY alkenes, but mainly alkenes.)

for monomers that’re longer than just one C=C, you need to be careful with how you condense it into the repeating unit:

condensation polymerisation

linking two monomers with different functional groups together by removing a small molecule, usually water. these means per linkage, one water molecule is formed.

polyesters

polyesters are formed by reacting a dicarboxylic acid with a dialcohol. (the di- prefix just means that the functional group is on both ends)

polyamides

polyamides can be formed by reacting a dicarboxylic acid with a diamine (functional group NH₂)

polypeptides are specific type of polyamides formed when amino acids undergo condensation polymerisation. they can do this because they have the functional group of both carboxylic acids and of amines:

Naturally occuring polymers

DNA is a polymer essential for life.

it encodes genetic instructions for the development and functioning of organisms

they consist of 4 different monomers called nucleotides. the nucleotide consists of a pentose sugar, a phosphate and a base - A,T, C or G.

the nucleotides bind by polymerisation into strands that then intertwine to produce the double helix shape.