An introduction to organic chemistry Flashcards
List and define the types of formula used to represent organic molecules
- Empirical formula: the formula that tells us the simplest ratio of the different atoms present in a molecule, e.g. CH2
- Molecular formula: the formula that tells us the actual numbers of each type of atom in a molecule, e.g.C3H6
- Structural formula: the formula that tells us about the atoms bonded to each carbon atom in an organic molecule, e.g. CH3CH=CH2
- Displayed formula: the drawing of a molecule that shows all the atoms and bonds within the molecule
- Skeletal formula: a simplified version of the displayed formula that has all the symbols for carbon andd hydrogen atoms removed, as well as the carbon to hydrogen bonds. The carbon to carbon bonds are left in place as are the bonds to other atoms. (shown below)
Define
general formula
A formula that represents a homologous series of compounds using letters and numbers, e.g. for alkanes C<em>n</em>H2n+2. By substituting a number for n in the general formula you get the molecular formula of a particular compound in that homologous series.
Define
aliphatic and aromatic
Aliphatic: an open chain compound
Aromatic: containing at least one benzene ring (shown below)
Define
functional group
An atom or group of atoms in an organic molecule that determine the characteristic reactions of a homologous series.
Explain homolytic fission
A type of covalent bond breaking in which both the atoms at each end of the bond leave with one electron from the pair that formed the covalent bond.
The species produced when a bond breaks homolytically is called a free radical: a very reactive atom or molecule that has a single unpaired electron.
Homolytic fission is denoted by half-headed arrows, free radicals are denoted by a dot (e.g. Cl•).
Homolytic fission requires energy, for example from UV light or high temperatures.
Explain heterolytic fission
A type of covalent bond breaking in which the more electronegative atom takes both the electrons in the covalent bond.
This produces an anion and a cation.
Heterolytic fission is denoted by a ‘curly’ arrow.
Define initiation, propagation and termination
initiation step: the first step in the mechanism of free-radical substitution of alkanes by halogens. It involves the breaking of the halogen-halogen bond by UV light from the Sun
propagation step: a step in a free-radical mechanism in which the radicals formed can then attack reactant molecules generating more free-radicals, and so on
termination step: the final step in a free-radical mechanism in which two free radicals react together to form a molecule
Define
nucleophile
A species that can act as a donor of a pair of electrons (they have a negative or partial negative charge)
- nucleo- “nucleus”; -phile “liking”
- high electron density
- anions e.g. carboanions, OH-, CN-, Br-
- molecules with lone pairs e.g. H2O, NH3, CH3OH
- moves to ‘attack’ electrophiles
Define
electrophile
A species that can act as an acceptor of a pair of electons in an organic mechanism (they have a positive or partial positive charge)
- electro- ‘electron’; -phile ‘liking’
- low electron density
- species with a vacant orbital at a relatively low energy level (so electrons can be accepted)
- cations e.g. carbocations, H+, H3O+
- polar molecules e.g. AlCl3, BF3, FeBr2
- doesn’t move: ‘attacked’ by nucleophile
List and define 5 types of organic reaction
- Addition: in which two reactant molecules combine to give a single product molecule
- Substitution: a reaction that involves the replacement of one atom, or group of atoms, by another
- Elimination: see condensation
- Hydrolysis: the breakdown of a compound by water, which is often speeded up by reacting with acid or alkali
- Condensation: in which two organic molecules join together and in the process eliminate a small molecule, such as water or hydrogen chloride
Explain oxidation and reduction in organic molecules
In organic reactions it is simpler to think of oxidation and reduction in terms of the number of oxygen/hydrogen atoms before and after a reaction.
- Oxidation: addition of oxygen, removal of hydrogen; [O] represents oxygen from the oxidising agent
- Reduction: removal of oxygen, addition of hydrogen; [H] represents hydrogen from reducing agent
Define
structural isomerism
and list its 3 types
Structural isomers have the same molecular formula but different structural formulae.
- Chain isomerism
- Positional isomerism
- Functional grouop isomerism
Describe chain isomerism
- different arrangements of the carbon skeleton
- can be straight or branched
- similar chemical properties
- slightly different physical properties
- lower m.p. & b.p.
- more branching means less contact points between molecules
- ∴ weaker van der Waals’ forces
Describe positional isomerism
- same carbon skeleton
- same functional group
- functional group is in a different position, i.e. connected to a different carbon on the skeleton
- similar properties
Describe functional group isomerism
- different functional group
- different chemical & physical properties
- same general formula
- e.g.
- alkenes and cycloalkanes
- alcohols and ethers
- aldehydes and ketones
- carboxylic acids and esters
Describe geometrical isomerism
- also known as cis-trans isomerism
- found in alkenes due to the restricted rotation around π bond, i.e. C=C double bond
- doesn’t occur with C-C bonds as they have free rotation
- doesn’t occur when two similar groups/atoms are attached to the same carbon
- different physical properties (m.p. & b.p., density)
- slighty different chemical properties (rates of reactions)
- cis- when similar/heaviest groups/atoms are adjacent on the double bond
- trans- when similar/heaviest groups/atoms are across the double bond
C=C bonds have a σ bond and a π bond. Pi bonds are formed by the sideways overlap between p orbitals. If you tried to rotate the C=C bond, the p orbitals won’t line up anymore and so the pi bond is disrupted. This costs energy and only happens if the compound is heated strongly. C-C bonds are single σ bonds, and can be rotated.
Describe optical isomerism
- optical isomers / enantionmers are mirror images that cannot be superimposed
- occurs when compounds have an asymmetric carbon atom, i.e when 4 different groups are attached to the same carbon
- this carbon atom is called the chiral centre
- differ in their reaction to plane-polarised light; rotates light in different directions
