10. Organic Chemistry

Fundamentals of Organic Chemistry

Naming Organic Compounds1. Prefix: longest continuous carbon chain 1: meth- 2: eth- 3: prop- 4: but- 5: pent- 6: hex- 7: hept- 8: oct-2. Identify type of bondsSingle: alkaneDouble: alkeneTriple: alkyne3. Suffix: Identify present functional groupsnone: -e-OH (hydroxyl): -ol-NH2 (amine): amino--X (halogen): chloro-, bromo- or iodo-(aldehyde): -al(ketone): -one(carboxyl): -oic acid(ester): -oateWhen there are multiple groups present, arrange them in alphabetical order prior to theroot name. 4. Identify the side chainsMethyl group (1 carbon)Ethyl group (2 carbons)...5. Use numbers and prefixes to indicate the number and locationof double/triple bonds or functional groups→ Number is indicated by a prefix:(mono-), di-, tri-, tetra-, penta-, hexa-→Location is indicated with a number, and multiple numbers are separated by a commaeg. 2,2-dimethyl-propane!! Always use lowest number possible (eg. 2 instead of 4 in 2-methyl-pentane)

Homologous seriesAlkanes→ general formula: C(n)H(2n+2)Alkyl radicals R- are alkanes with one of the hydrogens removed→ have a -1 chargeHomologous series have the same general formula with the neighbouring members of the series differing by CH2

Primary, Secondary and TertiaryPrimary→ Functional group is bonded to a carbon that is bonded to only one other carbon (R group)Secondary→ Functional group is bonded to a carbon that is bonded to two other carbons (R groups)Tertiary→ Functional group is bonded to a carbon that is bonded to three other carbons (R groups)

Class of organic compoundDifferent compounds that contain the same functional group aredivided into classes. → Alcohols→ Esters→ Aldehydes→ Ketonesetc.

Alkanes

Properties of AlkanesGeneral formulaC(n)H(2n+2)Low reactivity→ Strong and stable C-C and H-C bonds, low polarity→ Exceptions; undergo combustion and substitution reactions with halogens under UV lightCombust fully to give carbon dioxide and water→ Reaction is very exothermic; make good fuelsEven though chain alkanes are more common, they occur in ring structurescalled cycloalkanesAlkanes are fully saturated with hydrogens, contrary to alkenes and alkynes!Boiling point increases as carbon chain length increases

Free Radical SubstitutionAlkanes react with chlorine or other halogens in the presenceof UV light to form hydrogen chloride and a substituted alkane→ Bromine is only halogen that does not need UV light to initiate reactionReaction mechanism; photochemical homolytic fission→ heterolytic fission: both shared e go to one of the atoms→ homolytic fission: each bonded atom retains one e; creation of 2 free radicals (under influence of UV)→ Mechanism steps: 1. Initiation; homolytic fission of halogen 2. Propagation; free radical halogen reacts with alkane (eg. methane) to produce HX and a free radical methyl group 3. Termination; two free radicals react together (eg. radical methyl and radical chlorine)

Halogenalkanes→ more reactive than alkanes; can undergo nucleophilic substitution reactions

Alkenes and Alkynes

Properties of AlkenesSecond homologous series of hydrocarbonsGeneral formula: CnH2nContain at least one C=C bond→ (sigma + pi bond)→ sp2 hybridizationMore reactive than alkanes→ Undergo addition reactions→ Unsaturated hydrocarbonsCan be distinguished from alkanes using bromine water,or a halogen gas, which will decolourize in the presence of alkenesPhysical properties→ non-polar or weak polar→ no hydrogen bonding→ Relatively low mp/bp ~alkanes (Double bonds disrupt London forces)→ Water insoluble

Properties of AlkynesThird homologous series of hydrocarbonsGeneral formula: CnH2n-2Contain at least one triple carbon to carbon bond→ sigma + 2 pi bonds→ sp2 hybridizationUnsaturated hydrocarbons→ reactive like alkenes (can also perform additionreactionsPhysical properties→ Water insoluble→ Non-polar or weak polar→ Aroudnd the same boiling point of alkenes withthe same carbon chain lenght

Alcohols

Alcohols are organic compounds that contain a hydroxyl group (-OH)→ Compound names have the ending -ol→ OH group is often isolated in molecular formulaCombustion → Combust completely to give carbon dioxide and water→ Give out little pollution and doesn't contain sulphur→ Obtainable from renewable sourcesOxidizing agents→ Acidified potassium dichromate, K2Cr2O7→ Potassium permanganate, K2Mn2O7

Primary Alcohols OxidationIn primary alcohols, hydroxyl group is often found at the end of the chain→ methanol (wood alcohol), ethanol (alcohol in wine)→ oxidation products have lower boiling point than reactants; no hydrogen bonding1: Aldehyde→ eg. methanal (formaldehyde)→ Have a group with carbon with double bond to O and one bond to a H→ To obtain aldehydes, it can be distilled from the reaction mixture as soon as it is formed.→ Polar molecules - dipole-dipole interaction2: Carboxylic acid→ eg. methanoic acid (formic acid), ethanoic acid (vinegar)→ Have a C with double bond to O and OH (carboxyl group)

Secondary Alcohols Oxidation1st oxidation: Ketone→ eg. propanone→ have a C double bonded to an O in the middle of the chain→ Polar molecules - dipole-dipole interactions! Ketones cannot undergo further oxidation

Tertiary Alcohols OxidationAny further oxidation unlikely with oxidizing agent, except for combustion

Hydroxyl substitution reactionsAlcohols can be made using a halogenalkane and -OH group→ Halogen will be substituted by the hydroxyl groupeg. BrC2H7 + NaOH → C2H7OH + NaBr

EstersAn alcohol and an organic acid will react to an ester in a condensation reaction→ Performed in water bath + concentrated sulfuric acid catalyst→ Esters are compounds that often give a certain smell or taste→ Polar molecules

Structural isomers and Stereoisomerism

Structural isomers→ Compounds with the same molecular formula but a different structural formula→ Similar chemical properties but possibly different physical properties. ! The longer the carbon chain; the more structural isomers possible

StereoisomerismCis-trans isomerism→ Specific case of E/Z isomerism→ Occur when rotation around bond is restricted or prevented (eg. double/triple C-C bonds, pi-bond) when R1 and R2 are found on either side of the double bond→ Cis: both R groups on same side of double bond (up-up / down-down)→ Trans: R groups on opposite sides of double bond (up-down / down-up) → Can also occur in cycloalkanesE/Z isomerism→ Occurs in every case where where free rotation around a C=C bond is restricted, could have 4 different R groups→ Cahn-Ingold-Prelog (CIP) rules for determining the priority of the atoms/groups attached to the two carbons→ Z: both highest priority groups on same side (like cis, zusammen) E: higher priorities lie on opposite sides (like trans, entgegen)Physical and chemical properties of geometric stereoisomers→ Same chemical but different physical properties, eg. boiling pointOptical Isomerism→ All compounds that contain at leas one asymmetric or chiral carbon within the molecule; contains 4 different atoms or groups. → This carbon is known as the stereocentre→ Multiple stereocentres; multiple stereoisomers possibleEnantiomers: mirror imagesDiastereomers: not mirror imagesEnantiomers can rotate the plane of plane-polarized light→ One turns it to the left, the other to the right→ If a substance containes only the same enantiomers, the substance will be optically active - can be detected by a polarimeter

Polymerization

Polymer→ a long chain molecule produced by repeated chemical linkage (i.e. polymerization) of many small molecules (monomers)There are two types of polymers;1. Addition polymers→ formed by addition reaction of alkenes2. Condensation polymers→ formed by condensation reaction

Addition PolymersAddition reaction→ weak pi bond of alkene (C=C) monomer 'opens' allowing many (n) C-C units to form long chains→ Occur with catalyst + high pressure and temperature Peroxides (free radical mechanism) or Zeigler catalyst (carbocation mechanism)→ Most plastics are addition polymers, but little to no biological systems know how to break down these polymersAddition polymer properties→ Alkane-like→ Nonreactive (except in combustion)→ Non-biodegradable: cause pollution and environmental harm

Condensation PolymersCondensation reaction→ Elimination of simple molecules (H2O, HCl)→ Conditions: catalyst + water bathdicarboxylic acid + diol → polyester + H2Odiacylchloride + diamine → polyamine + HClCondensation polymers→ 2 functional groups needed per monomer; one at each end of a chain structure

Polyesters→ Form very long chains; fibres used for clothing→ n Diol + n dicarboxylic acid → [polyester]n + (n-1) H2O

Polyamides→ condensation reaction: n dicarboxylic acid + n diamine → [polyamide]n + (n-1) H2O→ form long-chain fibres used for clothingNote: ester + amide bonds are hydrolysable→ more biodegradable than addition polymersie. polyester + (n-1) H2O → n diacid + n diol→ Will degrade in nature but release organicresidues into environment

Condensation polymer examplesPoly(lactic acid), PLA→ Made from a single monomer, lactic acid→ Renewable resource; can be obtained through fermentation of starch or sugar, biodegradable→ Can undergo repeated condensation reaction with itselfKEVLAR→ Used for bulletproof vests→ a polyamide (aramid) with low density and very high tensile strength; used for body armour, sports equipment, brake linings, ropes

Nucleophilic Substitution Reactions

Key pattern → one bond breaks at C, other bond forms at CIn nucleophilic substitution reactions, there are 2 types:Sn1 → waits for the other atom to leaveSn2 → attacks and kicks off other atomNucleophile→ Groups with electron-rich atoms able to donate a pair of electrons (lewis bases)→ Ion/molecule which is strongly attracted to a region of positive charge. They are fully negative or have a strong negative charge somewhere.→ Examples: -OH hydroxides -CN cyanides H2O water (weak) NH3 ammonia (weak) HBr halogenacids

Sn1 ReactionsIn Sn1, the nucleophile 'waits' for the bonded halogen group toleave before reacting with the remaining carbocation→ 2 step reaction: 1. Leaving of halogen - slow 2. nucleophile bonding with carbocation - fast→ Can happen from both sides; 2 optical isomers formed Product is not optically active

Sn2 ReactionsIn Sn2 reactions, the nucleophile 'attacks' from the opposite side of themolecule and forces halogen group to leave→ Only 1 optical isomer is formed; product is optically active enantiomer is inverse of reactant→ X leaves at the same time as the nucleophile bonds; 1 step reaction with transition state

Factors affecting Sn1 or Sn2The 'Big Barrier'→ key factor that prevents the reaction from happening Sn1: carbocation stability increases as there are more bonds on the central carbon 3 > 2 >> 1 Sn2: steric hindrance Nucleophile has to attack from opposite side 1 > 2 >> 3Solvent characteristics→ Sovents can be nonpolar, polar protic or polar aprotic polar protic solvents: contain an -OH group, which makes them effective in stabilizing the carbocation > Sn1 is favoured, Sn2 disfavouredNucleophile type→ Weak nucleophile: Sn1→ Strong nucleophile: Sn2Rate law→ Sn1 is a 2 step reaction where the first step is the slow step; Rate law depends on substrate only→ Sn2 is a single step reaction Rate law depends on both substrate and nucleophile

Elimination reactions

Key pattern → substrate with a base forms an alkene + conjugateacid + leaving group

Factors affecting elimination or substitutionTemperature→ At lower temperatures, nucleophilic substitution occurs→ At higher temperatures, elimination will occur→ Often a combination between these two, but the higher T, the more elimination productsExplanation: Substitution forms 2 products while elimination forms 3products → greater increase in entropyWe have that G = H - TS At higher temperatures, Gibbs Free energy is more negativefor elimination reactions than substitution

Electrophilic Addition and Substitution Reactions

Electrophilic AdditionKey pattern → a pi bond is brokenElectrophile→ attracted to a region of high electron density

Addition to asymmetrical alkenesIn theory, two isomers could be produced in anaddition to an asymmetrical alkene. However, there is often one that is much more favoured.Markovnikov's observation→ There is a preference for secondary halogenated alkanes→ The hydrogen goes to the C with the most hydrogens bonded to itMarkovnikov's rule'In an addition reaction of a protic acid HX to an alkeneor an alkyne, the hydrogen atom becomes bonded tothe carbon that had the greatest number of hydrogensExplanation: Stability of carbocation when pi-bond breaks:3 > 2 >> 1Akyll groups tend to push electrons towards the carbonatom they are bonded to: stabilizes charge on carbocation

Electrophilic SubstitutionBenzene is the simpelest aromatic hydrocarbon compound (arene)and has a delocalized structure of pi-bonds around its ring> each C-C bond has order of 1.5> Susceptible to attack by electrophiles> Does not readily undergo addition but does substitutionNitration of BenzeneBenzene reacts with a mixture of concentrated nitric acid andconcentrated sulfuric acid when warmed at 50oC to give nitrobenzeneand water