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Basic concepts of organic chemistry (Structural isomerism (• Isomerism …
Basic concepts of organic chemistry
Organic chemistry
What is organic chemistry
• Originally chemistry of compounds derived from living systems
• Now defined as the
study of the structure, properties, composition, reactions and preparation of carbon-containing compounds
• Carbon in group 14 so contains 4 outer electrons – therefore each carbon atom can form 4 single covalent bonds to other atoms
Hydrocarbons
•
A compound containing only carbon and hydrogen
- for example alkanes and alkenes
•
Saturated
hydrocarbon has only single covalent bonds
•
Unsaturated
hydrocarbon contains a double bond or triple bond (C-C multiple bonds) – for example ethene is unsaturated
• A
homologous series
is a family of compounds with similar chemical properties and whose successive members differ by a –CH2- group
• A
functional group
is the part of the organic molecule that is largely responsible for the molecules chemical properties
Nomenclature of organic compounds
Naming hydrocarbons
•
Aliphatic
- carbon atoms are joined to each other in straight or branched chains, or non-aromatic rings
•
Alicyclic
- carbon atoms are joined to each other in ring structures, with or without branches
•
Aromatic
- some or all of the carbon atoms are found in a benzene ring
•
Alkanes
- containing single carbon-carbon bonds
•
Alkenes
- containing at least 1 double carbon-carbon bond
•
Alkynes
- containing at least 1 triple carbon-carbon bond
Naming aliphatic alkanes
• Identify longest carbon chain and name it
• Identify any side chains attached to parent chain – known as alkyl groups and add their name as a prefix to the parent chain
• Add numbers before any alkyl groups to show their position
• When the5re are two or more possible chains of same length, the chain with the most branches is considered the longest chain
• For
alicyclic
alkanes, add cyclo- in front of the parent chain
• See textbook for other functional groups
Representing formulae of organic compounds
Chemical formulae
•
Molecular formula
- shows number and type of atoms of each element present in a molecule but doesn’t show how the atoms are joined together and different molecules can have the same molecular formula
•
Empirical formula
- simplest whole-number ratio of the atoms of each element present in a compound
•
General formula
- the simplest algebraic formula for any member or a homologous series – can be used to generate any molecular formula
•
Displayed formula
- shows relative positioning of all of the atoms in a molecule and the bonds between them
•
Structural formula
- Uses smallest amount of detail necessary to show the arrangement of the atoms in a molecule eg. Ch3Ch2Ch2Ch3
Skeletal formula
• Simplified organic formula – can remove all of the carbon and hydrogen labels from carbon chains and can remove any bonds to hydrogen atoms
• A line represents a single bond, an intersection of two lines represents a carbon bond
• The end of a line represents a –CH3 group
• Used a lot in representing alicyclic and aromatic compounds
• When functional groups are present in a molecule they must be represented in the skeletal formula
Structural isomerism
•
Isomerism
- where different compounds have the same molecular formula
•
Chain isomerism
- carbon chain is arranged differently – for example one compound might be branched
•
Positional isomerism
- this is where the functional group is attached to a different carbon atom
•
Functional group isomerism
- where the compounds contain different functional groups so members of different functional homologous series – for example aldehydes and ketones
Introduction to reaction mechanisms
Types of bond fission
•
Homolytic fission
- when a covalent bond breaks each of the bonded atoms take
one
of the shared pair of electrons from the bond
• Each atom now has a single unpaired electron – the atom or group with the unpaired electron is a
radical
•
Heterolytic fission
- when a covalent bond breaks one of the atoms takes
both
unpaired electrons – the atom which takes becomes negative, and atom which loses becomes positive
Organic reaction mechanisms
• An equation tells you about reactants, products and stoichiometry (ratio of moles) of a reaction, but doesn’t tell you about how the reaction takes place
• The
reaction mechanism
tells us how the reaction happens
•
Curly arrows
used to show the movement of electron pairs when bonds are being broken or made
• A half curly arrow (half of the arrowhead) can be used to show the movement of a single unpaired electron
Types of reaction
•
Addition
- two reactants join together to form one product
•
substitution
- an atom or group of atoms is replaced by a different atom or group of atoms
•
Elimination
- involves the removal of a small molecule from a larger one: one reactant forms two products