OCR AS Chemistry Unit 2 (Module 1)

ISOMERISM 

Structural isomerism exists when molecules are composed of the same number and type of atoms but these atoms are arranged in different ways.

KEY DEFINITION: Structural Isomerism- are molecules with the same molecular formula but with different structural arrangements of atoms.

Structural isomerism can occur in 3 ways: 

• The hydrocarbon chain may be branched or unbranched.
• A functional group can be in different positions along the main hydrocaron chain.
• The functional groups may be different

In stereoisomerism the atoms making up the isomers are joined up in the same order but have different arrangements in space.

KEY DEFINTION: Stereoisomers- are compounds with the same structual formula, but with a different arrangement of the atoms in space.

E/Z Isomerism 

A molecule must satisfy two criteria to have E/Z isomerism

• a carbon-carbon double bond
• each carbon in the double bond must be attached to two different groups.

KEY DEFINTION: E/Z Isomerism- is a type of stereoisomerism in which different groups attached to each carbon of a carbon-carbon double bond maybe arranged differently in space because of the restricted rotation of the carbon-carbon double bond.

KEY DEFINTION: Cis-trans isomerism is a special type of E/Z isomerism in which there is a non-hydrogen group and a hydrogen on each carbon of a carbon-carbon double bond: the cis isomer (Z isomer) has the H atoms on each carbon on the same side; the trans isomer (E isomer) has the H atoms on each carbon on different sides.

Z= together

E= apart

Trans= groups on opposite sides; E

Cis= groups on the same side; Z

If you have a C=C, then think carefully about the possibility of cis=trans isomers.

ORGANIC REAGENTS AND THEIR REACTIONS

Bonds are broken by bond fission.

Covalent bonds can be broken in two ways:

• homolytic fission
• heterolytic fission

KEY DEFINITION: Homolytic fission- is the break of a covlent bond, with one of the bonded electrons going to each atom, forming two radicals X-Y --> X. + Y. 

A radical is a species with an unpaired electron.

KEY DEFINITION: Heterolytic fission is the breaking of a covalent bond with both of the bonded electrons going to one of the atoms, forming a cation (+ ion) and an anion (- ion) X-Y --> X+ + Y-

KEY DEFINITION: A nucleophile is an atom or group of atoms that is attracted to an electron deficient centre or atom, where it DONATES a pair of electrons to form a new covalent bond.

Nucleophiles are often positive negative ions with a lone pair of electrons.

Nucleophiles often include an electronegative atom with a lone pair of electrons and a delta-ve partial charge. Examples: :Br-  :OH-  H2O:  :NH3

KEY DEFINITION: An electrophile is an atom or group of atoms that is attracted to an electron rich centre or atom, which it accepts a par of electrons to form a new covalent bond.

Electrophiles are often positive ions. Some molecules can act as electrophiles, these often include an atom witha delta+ve partial charge. Examples: Br2, HBr, NO2+

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