Forensic Chemistry - Introduction to Organic Chemistry

Types of Formulae - 2

• Skeletal Formula

  • A skeletal formula is used to show a simplified organic formula by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional groups

  • Cyclohexane example given

• General Formula

  • Represents any member of a homologous series, for alkanes it is $C<em>nH</em>{2n+2}$

  • Examples: $CH<em>4, C</em>2H<em>6, … C</em>{99}H_{200}$

  • The formula does not apply to cyclic compounds such as cyclohexane ($C<em>6H</em>{12}$) - by joining the atoms in a ring, you need fewer H’s

Homologous Series

• A series of compounds of similar structure in which each member differs from the next by a common repeating unit, $CH_2$.

• Series members are called homologues and…

  • All share the same general formula.

  • Formula of a homologue differs from its neighbor by $CH_2$.

  • Contain the same functional group.

  • Have similar chemical properties (e.g., $CH<em>4, C</em>2H_6$, … etc.).

  • Show a gradual change in physical properties as molar mass increases.

  • Can usually be prepared by similar methods.

• Alcohols - First three members of the series:

  • $CH_3OH$ - Methanol

  • $C<em>2H</em>5OH$ - Ethanol

  • $C<em>3H</em>7OH$ - Propan-1-ol

Functional Groups

• Organic chemistry is a vast subject, so it is easier to split it into small sections for study.

• This is done by studying compounds which behave in a similar way because they have a particular atom, or group of atoms, FUNCTIONAL GROUP, in their structure.

• Functional groups can consist of one atom, a group of atoms, or multiple bonds between carbon atoms.

• Each functional group has its own distinctive properties, which means that the properties of a compound are governed by the functional group(s) in it.

• Examples:

  • Carbon skeleton with an alcohol group (-OH).

  • Carbon skeleton with an amine group (-NH2).

Common Functional Groups

• Table of common functional groups, their endings, general formulas, and examples:

  • Alkane: -ane, RH, $C<em>2H</em>6$ (ethane)

  • Alkene: -ene, , $C<em>2H</em>4$ (ethene)

  • Alkyne: -yne, , $C<em>2H</em>2$ (ethyne)

  • Haloalkane: halo-, RX, $C<em>2H</em>5Cl$ (chloroethane)

  • Alcohol: -ol, ROH, $C<em>2H</em>5OH$ (ethanol)

  • Aldehyde: -al, RCHO, $CH_3CHO$ (ethanal)

  • Ketone: -one, RCOR, $CH<em>3COCH</em>3$ (propanone)

  • Carboxylic Acid: -oic acid, RCOOH, $CH_3COOH$ (ethanoic acid)

  • Acyl Chloride: -oyl chloride, RCOCl, $CH_3COCl$ (ethanoyl chloride)

  • Amide: -amide, $RCONH<em>2$, $CH</em>3CONH_2$ (ethanamide)

  • Ester: -yl -oate, RCOOR, $CH<em>3COOCH</em>3$ (methyl ethanoate)

  • Nitrile: -nitrile, RCN, $CH_3CN$ (ethanenitrile)

  • Amine: -amine, $RNH<em>2$, $CH</em>3NH_2$ (methylamine)

  • Nitro: nitro-, $RNO<em>2$, $CH</em>3NO_2$ (nitromethane)

  • Sulphonic Acid: -sulphonic acid, $RSO<em>3H$, $C</em>6H<em>5SO</em>3H$ (benzene sulphonic acid)

  • Ether: -oxy-ane, ROR, $C<em>2H</em>5OC<em>2H</em>5$ (ethoxyethane)

How Many Structures?

• Draw legitimate structures for each molecular formula and classify each one according to the functional group present.

• Not all the structures represent stable compounds.

• Carbon atoms have 4 covalent bonds surrounding them.

• Oxygen atoms have 2.

• Nitrogen atoms have 3.

• Hydrogen has 1.

• Halogen atoms have 1.

• Examples:

  • $C<em>2H</em>6$: One

  • $C<em>3H</em>9Br$: Two

  • $C<em>4H</em>8$: Five - 3 with C=C and 2 ring compounds with all C-C’s

  • $C<em>2H</em>6O$: TWO - 1 with C-O-C and 1 with C-O-H

  • $C<em>3H</em>6O$: SIX - 2 with C=O, 2 with C=C and 2 with rings

  • $C<em>2H</em>7N$: Two

  • $C<em>2H</em>4O_2$: SEVERAL - Only 2 are stable

  • $C<em>2H</em>3N$: Two

Nomenclature

• Ideally, a naming system should tell you everything about a structure without ambiguity.

• There are two types of naming system commonly found in organic chemistry:

  • Trivial: based on some property or historical aspect; the name tells you little about the structure.

  • Systematic: based on an agreed set of rules (I.U.P.A.C); exact structure can be found from the name (and vice-versa).

• Homologous Series

  • paraffin - alkane (methane, butane)

  • olefin - alkene (ethene, butene)

  • fatty acid - alkanoic (carboxylic) acid (ethanoic acid)

• Individual Compounds

  • methane (methu = wine (Gk.))

  • butane (butyrum = butter (Lat.))

  • acetic acid (acetum = vinegar (Lat.))

I.U.P.A.C. Nomenclature

• A systematic name has two main parts.

  • STEM - number of carbon atoms in the longest chain bearing the functional group + a prefix showing the position and identity of any side-chain substituents.

  • Apart from the first four, which have trivial names, the number of carbons atoms is indicated by a prefix derived from the Greek numbering system. The list of alkanes demonstrate the use of prefixes. The ending -ane is the same as they are all alkanes.

  • Prefix C atoms Alkane

    • meth- 1 methane

    • eth- 2 ethane

    • prop- 3 propane

    • but- 4 butane

    • pent- 5 pentane

    • hex- 6 hexane

    • hept- 7 heptane

    • oct- 8 octane

    • non- 9 nonane

    • dec- 10 decane

• Working out which is the longest chain can pose a problem with larger molecules.

I.U.P.A.C. Nomenclature - How Long is a Chain?

• Because organic molecules are three-dimensional and paper is two-dimensional it can be confusing when comparing molecules.

• This is because…

  1. It is too complicated to draw molecules with the correct bond angles

  2. Single covalent bonds are free to rotate

• All the following written structures are of the same molecule - PENTANE $C<em>5H</em>{12}$

• A simple way to check is to run a finger along the chain and see how many carbon atoms can be covered without reversing direction or taking the finger off the page.

• In all the above there are… FIVE CARBON ATOMS IN A LINE.

I.U.P.A.C. Nomenclature Substituents

• Many compounds have substituents (additional atoms, or groups) attached to the chain.

• Their position is numbered.

• A systematic name has two main parts.

  • SUFFIX - An ending that tells you which functional group is present

    • See if any functional groups are present.

    • Add relevant ending to the basic stem.

    • In many cases the position of the functional group must be given to avoid any ambiguity

    • Functional group Suffix

      • Alkane - ANE

      • Alkene - ENE

      • Alkyne - YNE

      • Alcohol - OL

      • Aldehyde - AL

      • Ketone - ONE

      • Acid - OIC ACID

• 1-Chlorobutane

• 2-Chlorobutane

I.U.P.A.C. Nomenclature - Side-Chain

• Carbon-based substituents are named before the chain name.

• They have the prefix -yl added to the basic stem (e.g., $CH_3$ is methyl).

• Number the principal chain from one end to give the lowest numbers.

• Side-chain names appear in alphabetical order: butyl, ethyl, methyl, propyl.

• Each side-chain is given its own number.

• If identical side-chains appear more than once, prefix with di, tri, tetra, penta, hexa.

• Numbers are separated from names by a HYPHEN.

• Numbers are separated from numbers by a COMMA.

  • e.g. 2-methylheptane

  • e.g. 2,3-dimethylbutane

• Alkyl radicals

  • methyl: $CH_3$

  • ethyl: $CH<em>3-CH</em>2$ or $C<em>2H</em>5$

  • propyl: $CH<em>3-CH</em>2-CH<em>2$ or $C</em>3H_7$

I.U.P.A.C. Nomenclature - Apply the Rules and Name

Naming Alkenes

• Length

  • In alkenes, the principal chain is not always the longest chain

  • It must contain the double bond the name ends in -ENE

• Position

  • Count from one end as with alkanes.

  • Indicated by the lower numbered carbon atom on one end of the C=C bond

  • $CH<em>3CH</em>2CH=CHCH_3$ is pent-2-ene (NOT pent-3-ene)

• Side-chain

  • Similar to alkanes position is based on the number allocated to the double bond

  • $CH<em>2 = CH(CH</em>3)CH<em>2CH</em>3$ 2-methylbut-1-ene

  • $CH<em>2 = CHCH(CH</em>3)CH_3$ 3-methylbut-1-ene

Investigating Molecules

• Chemical

  • Chemical reactions can identify the functional group(s) present.

• Spectroscopy

  • IR detects bond types due to absorbance of i.r. radiation

  • NMR gives information about the position and relative numbers of hydrogen atoms present in a molecule

• Confirmation

  • By comparison of IR or NMR spectra and mass spectrometry.