Organic Chemistry II: Hydrocarbons and Formulas

• Organic chemistry II covers A2 organic naming and reactions

Basic Definitions

• Hydrocarbon: A compound of hydrogen and carbon only.
• Saturated: Contains single carbon-carbon bonds only.
• Unsaturated: Contains a C=C double bond.
• Molecular formula: Shows the actual number of each type of atom.
• Empirical formula: Simplest whole number ratio of atoms of each element in the compound.
• General formula: Algebraic formula for a homologous series, e.g., CnH2n.
• Displayed formula: Shows all covalent bonds present in a molecule.
• Homologous series: Families of organic compounds with the same functional group and general formula.
  • Gradual change in physical properties, such as boiling point, is apparent.
  • Each member differs by CH2 from the last member.
  • Similar chemical properties are observed.
• Functional group: Atom or group of atoms that causes similar chemical properties when present in different molecules.

Drawing Displayed Formulae

• Hydrogen atoms should be added such that each carbon has 4 bonds when drawing organic compounds.
• The shape around a carbon atom is tetrahedral, with a bond angle of 109.5°.

Rules for Naming Carbon Chains

• Count the longest carbon chain and name it appropriately.
• Find any branched chains, count their carbons, and add the appropriate prefix for each branch chain (e.g., -CH3 methyl, -C2H5 ethyl, -C3H7 propyl).
• When compounds contain more than one functional group, the order of precedence determines prefix or suffix forms.
• Highest precedence group takes the suffix, with others taking the prefix form.
• Double and triple bonds only take suffix form.
• Order of priority (highest first): Carboxylic acids > carboxylic acid derivatives > nitriles > aldehydes > ketones > alcohols > amines.

General Rules for Naming Functional Groups

• The functional group is indicated by a prefix or suffix (e.g., chloroethane).
• When using a suffix:
  • Remove the -e from the stem alkane name if the suffix starts with a vowel (e.g., propan-1-ol, butan-1-amine, ethanoic acid).
  • Do not remove the -e if the suffix starts with a consonant or if "di" or "tri" indicates multiple functional groups (e.g., propanenitrile, ethane-1,2-diol).
• The position of the functional group on the carbon chain is given by a number, with the lowest possible number assigned.
• For aldehydes, carboxylic acids, and nitriles, the functional group is always on carbon 1.
• Include numbers only if needed to avoid ambiguity.
• Functional groups take precedence over branched chains in giving the lowest number.
• Use di-, tri-, or tetra- when there are multiple identical groups, noting the addition of 'e' to the stem.
• Words are separated by numbers with dashes, and numbers are separated by commas.
• List multiple functional groups or side chains in alphabetical order (ignoring "di," "tri," etc.).
• The suffix for alkenes can precede other suffixes.

Halogenoalkanes

• Halogens are substituents on the carbon chain with prefixes -fluoro, -chloro, -bromo, or -iodo, including position numbers when necessary.

Alcohols

• Alcohols have the ending -ol, with position numbers added between the name stem and -ol when necessary.
• If a compound has an -OH group along with other functional groups needing a suffix, use the prefix hydroxy- for the -OH group.
• When there are multiple -OH groups, di- or tri- prefixes are used, adding the 'e' to the stem name.

Aldehydes

• The name of an aldehyde ends in -al.
• Because the C=O bond is always on the first carbon, no extra number is needed.
• If there are two aldehyde groups, di is put before -al, and an e is added to the stem.

Ketones

• Ketones end in -one.
• A number is needed to show the position of the double bond for ketones with 5 or more carbons.
• If there are two ketone groups, di is put before -one, and an e is added to the stem.
• The prefix oxo- should be used for compounds containing a ketone group and a carboxylic acid.

Carboxylic Acids

• These have the -oic acid ending, but no number is necessary for the acid group as it must be at the end of the chain.
• The numbering starts from the carboxylic acid end.
• Dioic acid describes chains with carboxylic acid groups on both ends.

Nitriles

• These end in -nitrile, with the C in the CN group counted as the first carbon.
• Note that the stem name is different (e.g., butanenitrile, not butannitrile).

Esters

• Esters' names have two parts.
• The part ending in -yl comes from the alcohol next to the single-bonded oxygen.
• The part ending in -anoate comes from the carboxylic acid (the chain including the C=O bond).

Acyl Chlorides

• Add -oyl chloride to the stem name.

Amides

• Add -amide to the stem name.
• Secondary and tertiary amides are named to indicate the two (or three) carbon chains; the smaller alkyl group gets -N, working like a number indicating a side alkyl chain.

Isomers

• Structural isomers: Same molecular formula, different structures (or structural formulae).
• Chain isomerism
• Position isomerism
• Functional group isomerism

Chain Isomers

• Chain isomers: Compounds with the same molecular formula but different structures of the carbon skeleton.

Position Isomers

• Position isomers: Compounds with the same molecular formula but different structures due to different positions of the same functional group on the same carbon skeleton.

Functional Group Isomers

• Functional group isomers: Compounds with the same molecular formula but with atoms arranges to give different functional groups.
• Aldehydes and ketones of the same chain length can be functional group isomers, (e.g. Propanal and propanone (both C3H6O)).

Stereoisomers

• Stereoisomerism: Same structural formula, different spatial arrangement of atoms.
• There are two types of stereoisomerism: geometrical (E- Z isomerism) and optical isomerism.

E-Z Stereoisomerism

• Alkenes can exhibit a type of isomerism called E-Z stereoisomerism.
• E-Z isomers exist due to restricted rotation about the C=C bond
• Single carbon-carbon covalent bonds can easily rotate
• E-Z stereoisomers arise when:
  • There is restricted rotation around the C=C double bond.
  • There are two different groups/atoms attached both ends of the restricted double bond.

Naming E-Z Stereoisomers

• On both sides of the double bond determine the priority group
• PRIORITY Group: The atom with the bigger A, is classed as the priority atom
• If the priority atom is on the same side of the double bond it is labelled Z from the german zusammen (The Zame Zide!)
• If the priority atom is on the opposite side of the double bond it is labelled E from the german entgegen (The Epposite side!)

Optical Isomerism

• Optical isomerism occurs in carbon compounds with 4 different groups of atoms attached to a carbon (called an asymmetric carbon).
• These four groups are arranged tetrahedrally around the carbon.
• Two compounds that are optical isomers of each other are called enantiomers.
• Optical isomers have similar physical and chemical properties, but they rotate plane polarised light in different directions.
• One enantiomer rotates it in one direction and the other enantiomer rotates it by the same amount in the opposite direction.
• One optical isomer will rotate light clockwise (+)(called dextrorotatory). The other will rotate it anticlockwise(-)(called laevorotatory).
• A racemic mixture (a mixture of equal amounts of the two optical isomers) will not rotate plane-polarised light.
• A carbon atom that has four different groups attached is called a chiral (asymmetric) carbon atom
• A mixture containing a 50/50 mixture of the two isomers (enantiomers) is described as being a racemate or racemic mixture.
• Many naturally occurring molecules contain chiral C atoms, but are usually found in nature as a pure enantiomer
• Different systems of nomenclature are is existence for optical isomers. D/L or +/- are commonly used, but both have been superseded by the more useful and informative R/S system (this is not on the syllabus - for information only).

Chemical Reactions and Optical Isomers

• Racemates form when a trigonal planar reactant or intermediate is approached from both sides by an attacking species
• Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the trigonal planar carbonyl is approached from both sides by the HCN attacking species: results in the formation of a racemate
• Because a racemate forms there will be no optical activity in the products
• If the reactant was chiral then during the reaction the opposite enantiomer would form. The product will rotate light in the opposite direction to the reactant

Compounds with C=O group

• Carbonyls are compounds with a C=O bond. They can be either aldehydes or ketones
• If the C=O is on the end of the chain with an H attached it is an aldehyde.
• If the C=O is in the middle of the chain it is a ketone
• The name will end in -al
• The name will end in -one

Solubility in water

• The smaller carbonyls are soluble in water because they can form hydrogen bonds with water.
• Pure carbonyls cannot hydrogen bond, but bond instead by permanent dipole bonding.

Reactions of carbonyls

• The C=O bond is polarised because O is more electronegative than carbon. The positive carbon atom attracts nucleophiles.
• Key point: Aldehydes can be oxidised to carboxylic acids, but ketones cannot be oxidised.
• In comparison to the C=C bond in alkenes, the C=O is stronger and does not undergo addition reactions easily.
• This is in contrast to the electrophiles that are attracted to the C=C.

Oxidation Reactions

• Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols and aldehydes to oxidise.
• Aldehydes can also be oxidised using Fehling's solution or Tollen's Reagent. These are used as tests for the presence of aldehyde groups.
• RCHO + [O] → RCO2H
• Oxidation of Aldehydes: RCHO + [O] → RCOOH
• Observation: the orange dichromate ion (Cr2O72-) reduces to the green Cr 3+ ion

Tollen's reagent

• Reaction: aldehydes only are oxidised by Tollen's reagent into a carboxylic acid and the silver(I) ions are reduced to silver atoms
• Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no change.

Fehling's solution

• Reaction: aldehydes only are oxidised by Fehling's Solution into a carboxylic acid and the copper ions are reduced to copper(I) oxide.
• Observation: Aldehydes : Blue Cu 2+ ions in solution change to a red precipitate of Cu2O, Ketones do not react
• CH3CHO + 2Cu2+ + 2H2O → CH3COOH + Cu2O + 4H+

Reduction of carbonyls

• Reducing agents such as NaBH4 (sodium tetrahydridoborate) or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonyls to alcohols.
• H+ HH H-C-C-C + 2[H] → н-с-с-с-3-н HH H propanal HHH Propan-1-ol
• Reaction: carbonyl → hydroxynitrile
• When naming hydroxy nitriles the CN becomes part of the main chain

Reaction of carbonyls with iodine in presence of alkali

• Only carbonyls with a methyl group next to the C=O bond will do this reaction. Ethanal is the only aldehyde that reacts. More commonly is methyl ketones.
• This reaction is called the Iodoform test

Reaction with 2,4-dinitro phenylhydrazine

• Use 2,4-DNP to identify if the compound is a carbonyl. Then to differentiate an aldehyde from a ketone use Tollen's reagent.

Solubility in Water

• The carboxylic acid are only weak acids in water and only slightly dissociate, but they are strong enough to displace carbon dioxide from carbonates.
• The smaller carboxylic (up to C4) acids dissolve in water in all proportions but after this the solubility rapidly reduces. They dissolve because they can hydrogen bond to the water molecules.

Hydrogen bonding in solid ethanoic acid

• Hydrogen bonding between dimer in solid ethanoic acid

Delocalisation

• The delocalised ion has equal C-O bond lengths. If delocalisation did not occur, the C=O bond would be shorter than the C-O bond.

Strength of carboxylic acids

• Increasing chain length pushes electron density on to the COO- ion, making it more negative and less stable. This make the acid less strong.
• Electronegative chlorine atoms withdraw electron density from the COO- ion, making it less negative and more stable. This make the acid more strong.

Methods of preparing carboxylic acids

• Reagent: potassium dichromate(VI) solution and dilute sulfuric acid
• Full Oxidation of Primary Alcohols:
  • Propanoic acid less acidic than ethanoic acid
  • Chloroethanoic acid more acidic than ethanoic acid
  • Nitrile carboxylic
• Observation: the orange dichromate ion (Cr2O72-) reduces to the green Cr 3+ ion

The Reactions of Carboxylic Acids

• Carboxylic acids will be reduced to primary alcohols
• The effervescence caused by production of CO2 with carboxylic acids with solid Na2CO3 or aqueous NaHCO3 can be used as a functional group test for carboxylic acids
• H-C + [O] → -OH

Reaction of carboxylic acid with phosphorous (V) chloride

• This reaction with PCl5 (phosphorous(V)chloride) can be used as a test for carboxylic acids. You would observe misty fumes of HCl produced.

Esterification

• Carboxylic acids react with alcohols, in the presence of a strong acid catalyst, to form esters and water.
• Esters have two parts to their names, eg methyl propanoate.
  • This bit ending is -anoate
• comes from the carboxylic acid and includes the C in the C=O bond.
  • The bit ending is -yl
• Comes from the alcohol that has formed it and is next to the single bonded oxygen.
• The reaction is reversible. The reaction is quite slow and needs heating under reflux. Low yields (50% ish) are achieved. An acid catalyst (H₂SO₄) is needed.

Uses of Esters

• Esters are sweet smelling compounds that can be used in perfumes and flavourings.

Reaction with water esters can be hydrolysed

• This reaction is reversible and does not give a good yield of the products.Reagents and dilute acid HCI
• Reagents: dilute sodium hydroxide

Carboxylic acid derivatives: Acyl Chlorides

-The Cl group is classed as a good leaving groupThis makes acyl chlorides and acid anhydrides much more reactive than carboxylic acids and esters

Reaction with Water

• Change in functional group: acyl chloride → carboxylic acid
• Reagents: water
• Observation: Steamy white fumes of HCI are given off

Reduction of carbonyls with NaBH4 (sodium tetrahydridoborate)

• or LiAIH4 (Lithium terrahydridoaluminate) will reduce Carboryls to alcohols

Polyesters

• There are two types of polymerisation: addition and condensation -Condensation:Condensation Polymerisation

The Reactivity can be explained by the presence of polar-Bonds

• Which can attract attacking species

The esterification reaction is when a carboxylic acid and an alcohol react forming an ester/Water in the presence of catalyst H+/ Acid and heat

• This reaction is reversible
• The acyl chloride reacts with an alcohol to give an ester
• This is not reversible
• Forms HCL instead of water
• Acyl chlorides hydrolyse or react with Water to give a carboxylic acid and HCL
• Acyl chlorides React with ammonia in the cold (Ammonia acts Like a Nucleophile) to give amides. With alcohols esters products
• Can't be broken down any further
• Poly(Esters) This is an ester with a long chain, they are:Biodegradable-The ester bonds are fairly easy to break In condensation Polymerisation-There are two different monomers