Fundamental Principles of Organic Chemistry notes

MDCAT Chemistry

This chapter explains why carbon forms a very large number of compounds, how organic compounds are classified, and how functional groups determine their properties. It also covers hybridization, common organic compounds, and the main types of isomerism, including chain, position, functional group, geometrical and tautomeric isomerism.

Nature and Importance of Organic Chemistry

Organic chemistry is the study of carbon compounds, especially compounds containing carbon and hydrogen. Carbon also combines with oxygen, nitrogen, sulphur, phosphorus and halogens to form many compounds.

Carbon forms a very large number of compounds because of its tetravalency and catenation. It can form single, double and triple covalent bonds with itself and with other elements.

Tetravalency means that one carbon atom can form four covalent bonds.
Catenation is the self-linking property of an element through covalent bonds.
Carbon forms straight chains, branched chains and rings.
Carbon can form C-C single bonds, C=C double bonds and C≡C triple bonds.
The ability to form four covalent bonds is the primary reason for the large number of carbon compounds.
The first organic compound synthesised in the laboratory was urea, NH2CONH2, prepared by Friedrich Wohler.
Organic compounds are generally covalent and may have low melting and boiling points compared with many ionic compounds.

Classification of Organic Compounds

Organic compounds are classified according to the structure of their carbon skeleton and the groups attached to it. The main classes are acyclic compounds, cyclic compounds, aromatic compounds and heterocyclic compounds.

Acyclic compounds have open carbon chains. They may be straight-chain or branched-chain. Cyclic compounds contain a ring of atoms, so they are not acyclic.

Acyclic compounds are also called open-chain compounds.
Straight-chain compounds have carbon atoms joined in a continuous chain without branches.
Branched-chain compounds contain one or more carbon branches.
Cyclic compounds contain atoms joined in a closed ring.
Carbocyclic compounds have rings made only of carbon atoms.
Heterocyclic compounds contain at least one atom other than carbon in the ring, such as nitrogen, oxygen or sulphur.
Aromatic compounds contain one or more benzene-like rings with delocalised pi electrons.
Pyridine is a six-membered aromatic heterocyclic compound containing one nitrogen atom and is classified as an azine.

Hydrocarbons and Common Organic Compounds

Hydrocarbons contain only carbon and hydrogen. They are divided into saturated and unsaturated hydrocarbons. Alkanes are saturated, while alkenes and alkynes are unsaturated.

Aromatic hydrocarbons contain aromatic rings. Benzene is the simplest aromatic hydrocarbon. Its six carbon atoms form a planar ring, and each carbon is bonded to two other carbon atoms and one hydrogen atom.

Alkanes contain only carbon-carbon single bonds and have the general formula CnH2n+2 for open-chain compounds.
Alkenes contain at least one carbon-carbon double bond and have the general formula CnH2n for one open chain with one double bond.
Alkynes contain at least one carbon-carbon triple bond and have the general formula CnH2n-2 for one open chain with one triple bond.
Benzene has the formula C6H6.
Aniline is a benzene derivative containing an amino group, so its formula is C6H5NH2.
Benzyl alcohol has the formula C6H5CH2OH. Its hydroxyl group is attached to a CH2 group outside the benzene ring.
Tetraethyl lead was used as an anti-knock petrol additive.

Functional Groups

A functional group is an atom or group of atoms that gives an organic compound its characteristic chemical properties. Compounds containing the same functional group often show similar reactions.

The carbon skeleton is named as the parent structure, while the functional group is represented by a suffix or prefix according to IUPAC rules.

Alcohol group: -OH. Example: CH3CH2OH, ethanol.
Aldehyde group: -CHO. Example: HCHO, methanal.
Ketone group: >C=O between two carbon groups. Example: CH3COCH3, propanone.
Carboxylic acid group: -COOH. Example: CH3COOH, ethanoic acid.
Ester group: -COO-. Example: CH3COOCH3, methyl ethanoate.
Amine group: -NH2, -NHR or -NR2. Aniline contains an amino group attached to a benzene ring.
Nitrile group: -C≡N. The carbon of the nitrile group is included in the parent chain.
Aryl group is an aromatic ring group, such as C6H5-, derived from benzene by removal of one hydrogen atom.
A compound containing -OH attached to a saturated carbon is an alcohol. CH3CH2OH is an alcohol.
If a structure contains an aryl ring, a nitrile group and an -OH group attached to a carbon bonded to two other carbons, its groups include an aryl ring, nitrile and secondary alcohol.

Hybridization and Shapes of Organic Molecules

Hybridization is the mixing of atomic orbitals of similar energy to form new hybrid orbitals. The hybridization of carbon depends on the number and type of bonds around the carbon atom.

The shape of a molecule can be predicted from its hybridization. Single, double and triple bonds contain different combinations of sigma and pi bonds.

sp3 hybridization produces four equivalent orbitals and a tetrahedral shape with a bond angle of about 109.5 degrees.
Carbon in methane, CH4, is sp3 hybridized.
sp2 hybridization produces three hybrid orbitals in a trigonal planar arrangement with a bond angle of about 120 degrees.
Carbon in formaldehyde, HCHO, is sp2 hybridized because it contains a C=O double bond.
Every carbon atom in benzene is sp2 hybridized.
sp hybridization produces two hybrid orbitals in a linear arrangement with a bond angle of 180 degrees.
Carbon atoms in ethyne, HC≡CH, are sp hybridized.
The sigma bond is formed by direct overlap along the internuclear axis, while a pi bond is formed by sideways overlap of unhybridized p orbitals.

Isomerism and Its Main Types

Isomerism is the phenomenon in which compounds have the same molecular formula but different arrangements of atoms or different spatial arrangements. Such compounds are called isomers.

Structural isomerism results from different connectivity of atoms. The main structural types at this level are chain isomerism, position isomerism, functional group isomerism and tautomerism.

Chain isomerism results from different arrangements of the carbon skeleton.
Position isomerism results when the same functional group or multiple bond occupies different positions in the same carbon skeleton.
Functional group isomerism occurs when compounds have the same molecular formula but different functional groups.
Tautomerism results from the shifting of a proton from one atom to another, together with movement of a double bond in the same molecule.
Geometrical isomerism is caused by restricted rotation, usually around a carbon-carbon double bond or within a ring.
1-Butanol can show chain isomerism, position isomerism and functional group isomerism.
Position isomerism requires at least three carbon atoms, as two carbon atoms do not provide enough different positions for the same group in an ordinary open chain.
Cis-trans isomerism is a type of geometrical isomerism.

Examples of Structural and Geometrical Isomerism

The molecular formula C4H10O demonstrates several types of isomerism. Butan-1-ol and butan-2-ol have the same carbon skeleton but different positions of the hydroxyl group. 2-Methylpropan-1-ol has a different carbon skeleton.

Alcohols and ethers can also have the same molecular formula but different functional groups. For example, butanol and ethoxyethane have the formula C4H10O.

The chain isomers of pentane, C5H12, are three: pentane, 2-methylbutane and 2,2-dimethylpropane.
Pentane and 2-methylbutane differ in the arrangement of their carbon chains, so they are chain isomers.
Butan-1-ol and butan-2-ol are position isomers.
Butan-1-ol and ethoxyethane are functional group isomers.
In cis isomers, similar groups are present on the same side of a double bond.
In trans isomers, similar groups are present on opposite sides of a double bond.
2-Butene can exist as cis-2-butene and trans-2-butene.
A compound shows geometrical isomerism when each carbon of the C=C bond has two different groups attached to it.
Rotation around a carbon-carbon double bond is restricted because breaking the pi bond would be required.

Carbon Classification, Analysis and Related Organic Facts

Carbon atoms in an organic molecule may be classified according to the number of other carbon atoms directly attached to them. This classification is useful in understanding alcohols, alkyl halides and reaction behaviour.

Organic compounds are also studied by qualitative and quantitative analysis. Qualitative analysis identifies which components or elements are present, while quantitative analysis determines their amounts.

A primary carbon is directly bonded to one other carbon atom.
A secondary carbon is directly bonded to two other carbon atoms.
A tertiary carbon is directly bonded to three other carbon atoms.
A quaternary carbon is directly bonded to four other carbon atoms.
A primary alcohol has the hydroxyl-bearing carbon attached to one other carbon, except methanol, which has no carbon attached to it.
A secondary alcohol has the hydroxyl-bearing carbon attached to two other carbons.
A tertiary alcohol has the hydroxyl-bearing carbon attached to three other carbons.
Qualitative analysis is the identification of the components of a sample.
Carbohydrates include cellulose, glycogen and galactose.
Among simple amines, dimethylamine is more reactive towards HI than trimethylamine because of its greater availability for protonation and less steric hindrance.

Stoichiometric Application in Organic Reactions

Chemical equations show the mole ratio in which reactants combine. The limiting reactant is the reactant that is completely consumed first and determines the amount of product formed.

For the reaction A + 2B -> C, one mole of A requires two moles of B. If the available amounts do not match this ratio, the reactant present in the smaller required proportion limits the product.

In A + 2B -> C, the mole ratio A:B:C is 1:2:1.
Five moles of A would require ten moles of B.
Only eight moles of B are available, so B is the limiting reactant.
Eight moles of B react with four moles of A.
The amount of product formed is four moles of C, assuming complete reaction.
The excess reactant is A, because one mole of A remains after four moles react.
Always compare the available mole ratio with the balanced equation before calculating product.

Key terms

Organic chemistry: The branch of chemistry concerned mainly with carbon compounds.
Tetravalency: The ability of carbon to form four covalent bonds.
Catenation: The self-linking property of an element through covalent bonds.
Acyclic compound: An organic compound having an open chain rather than a ring.
Cyclic compound: An organic compound containing a closed ring of atoms.
Aromatic compound: A compound containing a stable ring system with delocalised pi electrons.
Functional group: An atom or group of atoms responsible for the characteristic reactions of an organic compound.
Hybridization: The mixing of atomic orbitals to form equivalent hybrid orbitals.
Isomerism: The existence of compounds with the same molecular formula but different structures or spatial arrangements.
Chain isomerism: Isomerism caused by different arrangements of the carbon skeleton.
Position isomerism: Isomerism caused by different positions of a functional group or multiple bond.
Functional group isomerism: Isomerism in which the same molecular formula represents different functional groups.
Tautomerism: Isomerism involving proton transfer and movement of a double bond within the same molecule.
Geometrical isomerism: Isomerism caused by restricted rotation that gives different spatial arrangements.
Cis isomer: A geometrical isomer in which similar groups lie on the same side of a double bond.
Trans isomer: A geometrical isomer in which similar groups lie on opposite sides of a double bond.
Qualitative analysis: The identification of the components present in a sample.
Limiting reactant: The reactant consumed first that determines the maximum amount of product.

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Chemistry shortcuts

Finding the limiting reactant and percentage composition

Convert every given mass or volume into moles first. The reactant that produces the least amount of the required product is the limiting reactant.

Write the balanced equation and calculate moles using n = mass/Mr.
Use the mole ratio to calculate the product. For percentage composition, use percentage = mass of element in one mole of compound divided by molar mass, multiplied by 100.
Example: Percentage of nitrogen in KNO3 = 14/101 × 100 = 13.86%.
Answer: 13.86% nitrogen.

Use gas volume at molar volume only when the gas conditions are stated or are standard conditions.

Using gas volume, pressure and temperature relations

At the same temperature and pressure, gas volume is directly proportional to the number of molecules. For changing conditions, use P1V1/T1 = P2V2/T2.

At constant temperature and pressure, divide or multiply the volume in the same ratio as the number of molecules.
Example: 10 mL H2 contains 2 × 10^3 molecules. Oxygen in 200 mL contains 20 × 2 × 10^3 = 4 × 10^4 molecules.
Answer: 4 × 10^4 molecules.
For a rigid container, increasing temperature increases molecular speed and mean free path if the gas remains in the same phase.

The direct volume to molecule ratio does not apply when temperature or pressure changes.

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