Structure 3.2: Functional groups: Classification of organic compounds

Structural formula

A representation of the molecule showing how the atoms are bonded to each other.

Full structural formula

Shows every bond and atom.

Condensed structural formula

Often omits bonds where they can be assumed, and groups atoms together.

Condensed structural formula of ethane

CH3CH3

A stereochemical formula

Attemps to show the relative positions of atoms and groups around a central carbon in 3 dimensions

Functional groups

Atoms, or groups of atoms, that are present in organic compounds and are responsible for a compound's physical properties and chemical reactivity.

What functional group do carboxylic acids contain

-COOH

Alkyne general formula

CnH2n-2

Alkyne functional group

Alkynyl

Alkene functional group

Alkenyl

Alcohol general formula

CnH2n+1OH

P.g.348-349

Reaction pathway

Each step involved a functional group interconversation

Condensation reaction

A molecule of water is eliminated and a new bond is formed between the acid group of one amino acid and the amoino group of the other.

Amide link

The bond substituded in a condensation reaction (N-H)

Peptide bond

The bond that is a substituted amide link

Dipeptide

Two amino acids bonded together

Tripeptide

Three amino acids bonded together

Homologous series

A series of organic compounds with the same functional group but with each successive member differing by CH2

Most volatile to least volatile

Alkane>halogenoalkane>aldehyde>ketone>alcohol>carboxylic acid

Increasing strength of intermolecular attraction

London (dispersion) force → dipole-dipole interaction → hydrogen bonding

Increasing boiling point

London (dispersion) force → dipole-dipole interaction → hydrogen bonding

Main intermolecular attraction in alkanes

London (dispersion) force

Main intermolecular attraction in halogenoalkanes, aldehydes and ketones

Dipole-dipole interaction

Main intermolecular attraction in alcohols and carboxylic acids

Hydrogen bonding

Stem

Name for the longest chain of carbon atoms (e.g. meth-, eth-, prop-, etc.)

Rules for IUPAC nomenclature

1. Identify the longest straight chain of carbon atoms

2. Identify the functional group

3. Identidy the side chains or substituent groups

Distinction between class and functional group

Class refers to the type of compound and functional group which refers to the site of reactivity in the molecule.

Functional group of amine

-NH2

Functional group of halogenoalkane

-F, -Cl, -Br, -I

When do esters form?

When the alkyl group of an alcohol replaces the hydrogen of a carboxylic acid in a condensation reaction.

Example of ester reaction

R-COOH + R'OH → R-COO-R' + H2O

Where does the stem comes from in esters?

The parent acid

What is the prefix for esters?

The alkyl group

The ester made from the reaction of ethanol with ethanoic acid

Ethyl ethanoate

What do ethers consist of

2 alkyl chains linked by an oxygen atom

When naming ethers

-The longer chain will be the stem and retains its alkane name

-The shorter chain is regarded as a substituenet and is given the prefix alkoxy

Methoxypropane

When two chains are the same length for ethers

One is assigned as the alkane stem and the other becomes the substituent

Ethoxyethane

Prefix

Position, number and name of substituents

Suffix

Class of compound determined by functional group

Structural isomers

Molecules that have the same molecular formula but different arrangements of the atoms. Because of their different structures, they also have different physical and chemical properties.

How does more branching affect the boiling point in an isomer?

Lowers the boiling point

Why does more branching affect the boiling point in an isomer?

It influences the strength of the London dispersion forces occurring between neighbouring molecules because it reduces the amount of surface contact. Branched-chain isomers have less contact with each other than their0single chain isomers resulting in weaker attractions between instantaneous and indeuced dipoles on neighbouring molecules with weaker overall London dispersion forces and lower boiling points.

Primary molecules

A primary carbon atom is attached to the functional group and also to at least two hydrogen atoms.

Secondary molecules

A secondary carbon atom is attached to the functional group and also to one hydrogen atom and two alkyl groups.

Tertiary molecules

A tertiary carbon atom is attached to the functional group and is also bonded to three alkyl groups and so has no hydrogen atoms.

Number of H atoms attacthed to central N in primary amines

2

Number of H atoms attacthed to central N in secondary amines

1

Number of H atoms attacthed to central N in tertiary amines

0

Number of C atoms attached to central N in primary amines

1

Number of C atoms attached to central N in secondary amines

2

Number of C atoms attached to central N in tertiary amines

3

Chlorobenzene

Phenylamine

Nitrobenzene

1,2-dibromobenzene

1,3-dibromobenzene

1,4-dibromobenzene

How are the numbers assigned to the ring positions of subsituted benzene

They are done so that the lowest possible are obtained

Isomerism

Compounds with same molecular formula but different arrangemetns of the atoms

Structural isomerism

Atoms and functional groups attached in different ways

Stereoisomerism

Different spatial arrangements of atoms in molecules

Configurational isomerism

Can be interconverted only by breaking covalent bonds

Conformational isomerism

Can be interconverted by free rotation of sigma bonds

Cis-trans isomerism

Exists where there is restricted rotation around atoms

Optical isomerism

Chirality exists where there is an asymmetric carbon atom

What do double bonds consist of?

One sigma bond and one pi bond, withthe pi bond forming by sideays overlap of two pi orbitals. The refernce plane is perpendicular to the sigma bonds and passes through the double bond.

Cis isomer

Refers to the isomer that has the same groups on the same side of the double bond/ring.

Trans isomer

Isomer that has the same groups on opposite sides, or across the reference plane.

\Cycloalkanes

Contain a ring of carbon atoms that restricts rotation. The bond angles are stranied from the tetrahedral angles in the parent alkane.

Cis isomer for cyclic molecules

Trans isomer for cyclic molecules

Chiral

A carbon atom attached to 4 different atoms or groups.

Alternative names for chirals

Asymmetric or a sttereocentre

Optical isomerism

When the four groups, arranged tetrahedrally around the carbon atom with bond angles of 109;5 are arranged in two different three-dimensional configurations which are mirror images of each other.

Enantiomers:

• identical physical properties except they rotate the plane of plane-polarised light in opposite directions

• chemical properties are same except when they react with with other chiral molecules

• mirror images

Non-superimposable

This means that the molecules cannot be placed on top of one another and give the same molecule.

Disastereomers

Molecuels that have different configurations at some, but not all, chiral centres. These molecules are not mirror images of each other.

Polarimeter

Measures the direction of rotation when a beam of plane-polarized light passes through a solution of optical isomers.

What happens if light is passed through a polarizer?

• plane polarised light is passed through a tube containing a solution of the enantiomer

• the plane of the plane-polarised light is rotated which then passes through the analyser

• the analyser is rotated until light passes through and the the direction of the rotation is measured

Analyser

This can be rotated until the light ppasses through it so the angle of rotation can be deduced.

How do separate solutions of enaniomers, at the same concentration, rotate plane-polarised light?

In equal amounts but in opposite directions.

Optically active

Capable of rotating the plane of polarized light

What is the racemic mixture of a chira compound contain?

Equal concentrations of the two optical isomers.

Optically inactive

A racemic that does not rotate light

What are naturaly occurring chiral molecules?

Optically active, so they exist as only one enantiomer.

When a racemic mixture is reacted with a single enantiomer of another compound

The two components of the mixture, the (+) and (-) enantiomers, react to produce different products which have different chemical properties that can be separated from each other relatively easily.

What happens when you out a racemic mixture through a polarimeter?

opposite rotations cancel ecahother out→ enantiomers are optically inactive

Resolution

A means by which the two enantiomers can be separated from a racemic mixture

Asymmetric synthesis

The process for the manufacture of a single enantiomer using a chiral catalyst.

Qualitative analysis

The detection of the presence but not the quantity of a subtance in a mixture; for example, forbidden substances in an athlete's blood.

Quantitative analysis

The measurement of the quantity of a particular substance in a mixture; for example, the alcohol levels in a driver's breather

Structural analysis

A description of how the atoms are arranged in molecular structures; for example, the determination of the structure of a naturally occuring or artifical product.

Mass spectrometry

Used to determine the relative atomic and molecular mass.

Infrared spectroscopy

Used to identify the bonds in a molecule.

Nuclear magnetic resonance spectroscopy

Used to show the chemical environment of certain isotopes in a molecule

THe ionisation process in the mass spectrometer

This involves an electron from an electron gun hitting the incident species and removing an electron:

X (g) + e → X + (g) + 2e-

The molecular ion, or parent ion

Formed when a molecule losses one electron but otherwise remains unchanged