Chemistry - Paper 1

Kekule's model

Benzene exists in two states depending on position of double bond. Switches between these positions so fast that bromine cannot be attracted to the C=C

Arguments against:

• enthalpy of hydrogenation is < 3 x cyclohexane (less energy + more stablisied)
• there is only one isomer for 1,2-dibromobenzene not two
• carbon-carbon bonds are between C-C and C=C

Delocalised model

BEnzene is a cyclic planar hexagonal ring. each carbon has a triganol planar shape (120). Final 2p electron forms sideways overlap formign a ring of electron density (delocalisation). This gives it increased stability (delocalisation energy). Does not take part in addition reaction = less stable. does not decolourise bromine water, react with strong acids, or halogens under normal conditions

Nitration - Benzene + HNO3 (sulphuric acid + 50, acids both conc.) --> nitrobenzene + water

Alkylation - Benzene + chloroalkane (halogen carrier) --> alkylbenzene +HCl

Acylation - Benzene + acyl chloride (halogen carrier) --> acylbenzene (ketone) +HCl

Halogenation - Benzene + X2 (halogen carrier + room t.) --> halogenobenxene + HBr

Halogenc carrier - used to create halogen ions 

AlCl3 or FeBr3 or Fe

Benzene doesn't reaction with bromine under sc. because the electron density is too low

Benzene ring with a hydoxyl group. Solid at rt. and slighlty slouble in water (because of OH)

Reactions:

• Water - phenol dissolves in water to form a weak acidic solution (C₆H­₅OH à C₆H₅O^- + H⁺).
• Sodium hydroxide - C₆H­₅OH +NaOH à C₆H₅O^-Na⁺ + H₂O (neutralisation)
• Sodium - C₆H­₅OH +Na à C₆H₅O^-Na⁺ + ½H₂
• Bromine - Rt. no catalyst C₆H­₅OH +3Br₂ à C₆H­₅(OH)Br₃ + 3HBr

Uses: antiseptics, detergents, surfactants, disinfectants, (sailicyic acid) make asprin, make azo dyes

Contain fucntion group C=O where O is more electronegative. Made by oxidising alcohols with acidified dichromate ions Cr₂O₇^2-. Or reduced by warming sodium borohydride (NaBH4) with water (solvent) into alcohols. 

Tests:

• 2,4-DNPH + methanol + sulphuric acid = Bradu's reagent - forms an orange/yellow ppt.
• Acidified potassium dichromate - Orange à Green
• NaOH (aq) + AgNO₃(aq) + dil.NH₃(aq) = Tollens reagnet - silver mirror

To identify carbonyl, use 2,4-DNPH derivative, recrytalise, measure melting point.

Recystilisation:

Dissolve in boiling solvent (ethanol), cool (air à ice bath), remove using vaccum filter (Buchner or Hirsch), dry. 

Carboxylic Acids - Weak acids - contain COOH group. 

Reactions:

• Metals:CH₃COOH + Na à CH₃COO^- Na⁺  + ½H₂ 
• Bases: CH₃COOH + KOH à CH₃COO^-K⁺ +H₂O
• Carbonates: 2CH₃COOH + Na₂CO₃ à 2HCOO^-Na⁺ + CO₂ + H₂O

Esters

Making

• Carboxylic acid + alcohol (conc. sulphuric acid) à Ester + water
• Acid anhydride + alcohol (warm) à Ester + carboxylic acid

Breaking

• Acid: reflux + dil. sulphuric/hydrochloric acid = same as reactants/ reversible
• Alkaline: reflux + aqueous alkaline (NaOH) = alcohol + sodium salt of carboxylic acid

Fats and oils are esters made from fatty acids and glycerol (propane-1,2,3 triol)

Cis unsaturated > trans unsaturatd > saturated

Cis unsaturated fats cannot pack closely together and so are liquids at rt

• HDLs carry chloesterol out of the blood to the liver
• LDLs deposit chloesterol onto the artery walls creating atheromas

Trans and saturated fats raise LDL levels and decrease HDL levels and so increase risk of heart disease.

Buodiseal is amde by reacting plant oils (palm or rapeseed) with ethanol or methanol to create ethyl or methyl fatty acids. This si done with an alklaine catalyst (NaOH) and produces glycerol which is sold to pharmaceutical companies.

Derivates of ammonia where the hydrogen atom(s) are replaced by R groups.

Amines act as weak bases becayse their lone pair of electrons can accept protons.

Azo Dyes

Daizotisation:

• Phenylamine + Nitrous acid NO₃ <10^o
• Nitroous acid is g=formed by sodium nitrate and excess HCl
• Forms diazonium salt and water 

Coupling:

• Diazonium salt + phenol under alkaline conditions 
• Azo dye (-N=N-)

Zwitter ions are when the acidic carboxylic acid and basic amine group interact to form an internal salt. The H from the carboxylyic acid is donated the the amine group, making the left positive and the right negative - there is no overall charge.

The isoelectric point is the pH at which an amino acid exists as a zwitterion

Reactions

• More acidic: behaves as a base, accepts a proton, becomes positive
• More alkaline: behaves as an acid, donates a proton, becomes negative

Polypeptides

Amino acids joined tofether in a condensation reaction to form peptide bonds.

Hydrolysis:

• Acid: moderatly conc. HCl (aq), reflux = positively charged amino acid
• Alkaline: NaOH(aq) >100 = sodium salt of amino acids

Stereoisomers is when atoms are joined up in the same order but have a different arrangemtn in space. Optical isomers are non-superimposable mirror images of each other and are called that becaue they have different effects on plane-polarised light. This occurs in organic molecules with a chiral carbon.

Pharmacy

When used in pharmacy optical isomers can have undesirable side effects and increase drug doses.Synthesis of chrical compounds produce a mixture of optical isomers - living systems only produce 1. 

Seperating chiral compouds is difficutl as they have simialr properties (melting/boiling/solubility)

Seperation involves:

• Enzymes or bacteria to promote stereoselectivity
• electrophoresis/chromatography as a seperation technique
• use of naturally chiral molecules to start with (left handed amino acids or sugars)

Condensation polymerisation involves joining tow monomers together with the elimination of water. It requires monomers wiht two different functional groups.

Polyesters

Made either from a dicaboxylic acid and a diol, or from a hydroxycarboxylic acid.

Hydrolysis:

• Acidic: hot aqeous acid = produces monomer units
• Basic: hot aqueous alkali (NaOH) = produces sodium salt of carboxylic acid and alcohol

Polyamindes 

Made from dicarboxylic acids and diamines or from amino acids

Hyrdrolysis:

• Acidic: hot aqueous acid (HCl) = ammonium salt of amine and carboxylic acid
• Basic: hot aqueous alkali (NaOH) = sodium salt of carboxylic acid and amine

Have chemical bonds that can undergo hydrolysis

Mainly based on polyesters

Photodegradbale polymers become weak and brittle when exposed to sunlight for prolongued periods - made by bledning polymer with light sensitive additives or by incorporating carbonyl bonds within the backbone.

Form waxy compounds when exposed to light and are then converted into CO₂ and water by bacteria

Used to seperate components in a mixture. Used for analysis of drugs, flavourings, forenstic evidence, air/water samples. 

Chromatorgaphy involves two phases:

• Mobile - moves in one definite direction
• Stationary - fixed in place, different affinities for different compounds and so alters their speed

Solid stationary phases seperate by adsorption and liquid stationary phases seperate by solubility. 

Stationary: thin layer of adsorbent (silical gel or alumina) coated on a flat inert support

Mobile: liquid solvent 

Steps:

• spot of dissolved sample placed on TLC plate and allowed to dry
• plate is placed ina  jar containing solvent and jar is sealed (saturate air with solvent vapours)
• solvent rises up stationary phase and carries components with it
• seperation is acheived by adsorption to plate
• once solvent reaches the top it is removed and left to dry, components may be developed with UV light

Rf values are the distance moved by compenet divided by solvent.

Limitations:

Some compounds have similar Rf values. Unknown compounds may have no Rf valyes. Difficult ot find a solvent that seperates all components.

Statioanry phase: thin layer of liquid or solide on the inside of capillary tubing 

Mobile phase: carrier gas (inert helium or nitrogen)

Steps:

• mixture is injected into gas chromatograph - vapourised
• mobile phase flushes through the column
• components are slowed by stationary phase (dissolve or adsorb)
• each component leaves column at different times- detected by computer

Retention time is the time for a compound to pass from inlet to detector

Area under each peak on chromatogram is proportional to amount of sample.

Limitations:

Thousands of chemicals may have the same renetion time, peak shape and detector response. Not all substances will be detected/seperated. Smalll samples can hide beneath others. Unknown samples will have no reference retention times for comparision.

TMS (tetramethylsilane) is used to give reference peak, as all atoms are int eh same enviroment.

Carried out in solution. Solvent must contain no hydrogen and deuterium, D (one proton, one neutron) instead. CDCl₃ is usally used.

Proton: peak area = proportion of protons. Splitting is caused by interaction of spin states of portons on adjacent carbons, calculated by n+1. 

OH/NH can occur over a wide rang of chemical shifts and don't normally split. Heavy water (D₂O) can be used as a sovlent, the Ds replace Hs and the OH/NH peaks disapear.

Carbon: peak size has no relevance. 

NMRs are used to check composition of organic compounds during drug synthesis.

MRIs are a type of NMR used to diagnose medical conditions. They are ahrmless and non-invasive and produce 3D NMR spectrum.