Functional groups: classification of organic compounds

empirical formula

• simplest whole number ratio of atoms in molecule

molecular formula

• actual number of atoms in a molecule

structural formula

• spatial arrangement of all atoms and bonds in a molecule

• condensed:

  • enough info is shown to make structure clear, but most bonds omitted

  • only important bonds are shown, e.g double/triple bonds

  • functional groups are shown using brackets

skeletal formula

• all of the carbon-carbon bonds represented by lines

  • end of each line and point where 2 lines meet = carbon atom

  • most hydrogen atoms removed except those part of functional group

stereochemical formula

• shows relative positions and 3D geometry of atoms and groups of atoms around chiral carbon

• standard convention:

  • bonds in plane of paper are drawn as solid lines

  • bonds coming forward out of plane towards you drawn as wedge

  • bonds going backwards out of plane are drawn as dashed wedge

functional groups

• groups of atoms found in organic compounds

homologous series

• a family of similar compounds, having the same functional group, therefore similar chemical properties

• successive members differ by CH₂

• have the same general formula

• have gradually changing physical properities

• as homologous group is ascended, size of molecule increases

alkanes

• C_nH_2n+2

• FG: alkyl

• suffix: -ane

alkenes

• C_nH_2n

• FG: alkenyl

• suffix: -ene

alkynes

• C_nH_2n–2

• FG: alkynyl (C=-C)

• suffix: -yne

halogenoalkane

• C_nH_2n+1X

• FG: halogeno

• prefix: fluoro ..etc

alcohol

• C_nH_2n+1OH

• FG: hydroxyl

• suffix: -hydroxy, -ol

aldehyde

• C_nH_2nO

• FG: carbonyl (C=OH)

• suffix: -al

• R-COH

ketone

• C_nH_2nO

• FG: carbonyl (C=O)

• suffix: one

• R-CO-R

carboxylic acid

• C_nH_2n+1COOH

• FG: carboxyl (C=OOH)

• suffix: -oic acid

ether

• C_nH_2n+2O

• FG: alkoxy (-O-)

amine

• C_nH_2n+1NH₂

• FG: amino (NH2)

• suffix: -amine

amide

• C_nH_2n+1NO

• FG: amido (C=ONH2)

• suffix: -amide

ester

• C_nH_2nO₂

• FG: ester (C=OO)

• suffix: -oate

physical trends in homologous series

• bpt & mpt increases with increased molecular size

  • each additional CH2 adds 8 more electrons, increasing strength of London forces

naming alkanes

• saturated

• alk + ane

  • alk depends on number of carbons in chain

• meth, eth, prop, but, pent, hex, hept, oct, non, dec

• if any side chains or functional groups, the position of these groups is indicated by numbering the carbon atoms in longest chain starting at the end that gives the lowest possible numbers in the name

  • hydrocarbon side chain shown by brackets in structural formula

  • side chain is named by adding -yl to normal alkane stem

  • if there are 1+ of the same alkyl side chain, di-, tri-

  • numbers separated from words by hyphen

  • if there are more than one type of alkyl side chain, side chains listed in alphabetical order

naming alkenes

• unsaturated

• named using alk + ene

• in straight chain of 4+ carbons, position of C=C double bond must be specified

  • carbon chains numbered starting with end closest to double bond

  • lowest numbered carbon atom participating in double bond is indicated just before the -ene

naming alkynes

• unsaturated

• named using alk + yne

• in straight chain of 4+ carbon, position of triple bond must be indicated

  • carbon chains numbered starting with end closest to triple bond

  • lowest numbered carbon atom participating in triple bond is indicated just before the -yne

naming halogenoalkanes

• named using prefix chloro-, bromo-, iodo- with ending -ane

• in straight chain of 3+ carbon atoms, position of halogen atom must be specified

  • carbon chains numbered starting with end closest to halogen

  • number of carbon atom attached to halogen is indicated just before the prefix

• when multiple functional groups, position and type must be given

naming alcohols

• alco + ol

  • if 2 OH groups present, it’s a diol

• in straight chain of 3+ carbon atoms, position of OH must be specified

  • carbon chains numbered starting with end closest to OH

  • number of carbon atom attached to OH is indicated just before the suffix

naming aldehydes

• if carbonyl group is on the end of a chain then it is an aldehyde and has functional group RCHO

• named using alkan + al

  • no need for numbers as aldehyde will always be on carbon 1

naming ketones

• ketones have a minimum of 3 carbons and have general functional group formula RCOR

• named using alkan + one

• after butanone, carbonyl group can have positional isomers so numbering must be used

naming carboxyl acids

• named using alkan + oic acid

• no need for numbers as carboxyl group always on carbon 1

isomer

• compounds that have the same molecular formula but different arrangement of atoms

structural isomers

• same molecular formula, different structural formula

• 3 types of structural isomerism:

  • functional group isomerism

  • positional isomerism

  • branched chain isomerism

functional group isomerism

• when diff functional groups result in the same molecular formula

• homologous series that can be functional group isomers of eachother:

  • alkenes and cycloalkanes

  • alcohols and ethers

  • aldehydes and ketones

positional isomerism

• differences in position of functional group in each isomer

• some organic compounds that can be described as having primary, secondary or tertiary structures will exhibit isomerism (alcohols and halogenoalkanes)

  • primary, secondary, tertiary relate to number of carbon atoms that the functional group carbon is attached to

branched chain isomerism

• same molecular formula, but their longest hydrocarbon chain is not the same

• caused by branching, where the longest hydrocarbon is broken into smaller pieces and these smaller pieces are added as side chains

• branching can only occur with 4+ carbon chains

isomerism in amines

• amines follow a different classification system of primary, seconday, tertiary to alcohols and halogenoalkanes

• classification based on number of alkyl groups attached to the nitrogen in the amine

  • primary: nitrogen attached to 1 other carbon atom (alkyl groups)

  • secondary: nitrogen attached to 2 other carbon atoms (alkyl groups)

stereoisomerism

• have the same structural formulas, but differ in their spatial arrangement

• 2 types of stereoisomers

  • conformational

  • configurational

    • cis/trans isomers

    • optical isomerism

conformational isomers

• occur in saturated compounds

  • due to free rotation about a single sigma bond

• free rotation allows easy interconversion from one isomer to the other

cis/tans isomers in alkenes

• occur in unsaturated compounds,

  • groups attached to C=C carbons remain fixed in position

    • due to presence of pi bond, free rotation isn’t possible

• cis isomers have 2 functional groups on the same side of double bond (both above or both below)

• trans isomers have 2 functional groups on opposite sides of double bond (1 above 1 below)

naming cis/trans isomers

• for cis/trans to exist, 2 different atoms/groups of atoms on either side of C=C bond are needed

• if there is more than one atom/group of atoms on either side of the C=C bond, the naming system fails

• works with 3 atoms but 2/3 atoms must be the same and on opposite sides of double bond

cis/trans isomers in cycloalkanes

• can also occur in cycloalkanes, as the C-C bond is part of a ring system, restricting rotation

• cis isomers occur when atoms are on same side of ring (both above or below)

• trans isomers occur when atoms are on opposite sides of the ring (1 above 1 below)

optical isomers

• chemicals that contain a chiral carbon

  • a carbon atom that has 4 different atoms/groups of atoms attached to it

  • carbon atom is asymmetric

• compounds with 1 chiral centre exist as a pair of optical isomers called enantiomers

• enantiomers are non-superimposable - mirror images of eachother

diastereomers

• compounds that contain more than one chiral centre

• are not mirror images of eachother as each chiral carbon has 2 isomers

• so have different physical and chemical properties

properties of optical isomers

• chemical

  • different behaviours in chiral environments

• physical

  • identical physical properties except they differ in ability to rotate the plane of polarised light

  • entantiomers are described as optically active

    • 1 enantiomer rotates plane polarised light in clockwise direction, the other in anticlockwise

    • rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance

racemic mixture

• a mixture containing equal amounts of each enantiomer

• typically optically inactive as the enantiomers will cancel out each other’s effect on plane polarised light

mass spec fragmentation patterns

• when compound analysed in mass spec, molecules bombarded with a beam of high speed electrons, knocking off some electrons from molecule forming molecular ions

• relative abundance of detected ions form mass spectrum

• the peak with the highest m/z value is the molecular ion (M+) peak. the value of m/z is the Mr of the compound

• mass spec values for particular fragments in DB

fragmentation patterns

• different compounds may have the same Mr, so to determine further, fragments that may appear are analysed as they are characteristic of certain molecules

alcohol fragmentation pattern

• tend to lose a water molecule giving rise to peak at 18, below the molecular ion

• another common peak found at m/z 31, corresponding to loss of CH₂OH⁺ fragment

IR interpretation

• covalent bonds vibrate in different ways, frequency of vibration occurs in IR region of EM spectrum

• if organic molecule is irradiated with IR energy that matches the natural vibration frequency of its bonds, it absorbs some of that energy and the amplitude of vibration increases - this is resonance

IR spectroscopy

• a technique used to identify compounds based on changes in vibrations of atoms when they absorb IR of certain frequencies

• spectrophotometer irradiates sample with IR radiation and detects intensity absorbed

• IR only absorbed if molecule has permanent dipole that changes as it vibrates

• resonance frequency is the specific frequency at which the bonds will vibrate

• IR spectrum shows wavenumbers (reciprocal of wavelength)

• characteristic absorptions can be matched to specific bonds in molecules

infrared spectroscopy and GH gases

• used to identify pollutants in vehicle emissions in the air

• used to measure alcohol levels using roadside breathalysers

  • IR is passed through breath, characteristic bonds of ethanol measured

proton NMR spectroscopy

• only atoms with odd mass numbers show signals on NMR spectra and have property of nuclear spin

• in ¹H NMR, magnetic field strengths of protons in organic compounds are measured and recorded on a spectrum

• samples are irradiated with radio frequency energy, and subjected to strong magnetic field

• protons on different parts of a molecule absorb and emit diff radio frequencies

chemical environments in NMR

• hydrogen atoms of an organic compound reside in different chemical environments

• e.g CH3OH has hydrogen in 2 chemical environments CH3 and OH

• protons in the same environment are chemically equivalent

main freatures of H NMR spectra

• number of different peaks

  • each proton in particular chemical environment absorbs at a particular frequency

  • number of peaks = number of different chemical environments

• area under peak

  • proportional to number of H atoms in that particular chemical environment

  • each area is integrated and heights of integrated traces can be used to obtain ratio of number of hydrogen atoms in each environment

• chemical shift

  • chemical shift of each absorption is measured in ppm relative to TMS, which has a 0 ppm

• splitting pattern

  • the chemical shift of protons within a molecule is slightly altered by protons bonded to adjacent carbon molecules

  • spin-spin coupling shows up in high res NMR as splitting patterns

  • if number of adjacent equivalent protons is n, the signal is split into n+1

TMS as reference standard

• tetramethylsilane is used because:

  • all protons in same environment so gives strong single signal

  • not toxic and unreactive, so doesn’t interfere w/ sample

  • volatile, so can be easily removed

peak splitting

• high resolution NMR gives more complex signals which appear to be split into sub-peaks - this is multiplicity

• splitting pattern of each peak is determined by number of protons in neighbouring environments

• if the spin of a neighbouring proton is aligned with the spin of the proton in question, the magnetic field is strengthened, resonance is stronger and chemical shift is higher

• if the spin of a neighbouring proton spins against the proton in question, the magnetic field is weakened, resonance is weaker and chemical shift is lower

• the resulting high res NMR peak splits into a doublet, 2 equal peaks

• when there are 2 neighbouring protons, 3 separate peaks obtained based on effect on magnetic field (stronger, unchanged, weaker), so a triplet is obtained (1:2:1)

• when there are 3 neighbouring protons, 4 separate peaks obtained based on effect on magnetic field, so quartet obtain (1:3:3:1)