Organic-Introduction

Organic chemistry: the chemistry of carbon and its compounds.

(which are central to life on Earth.)

3 important properties of carbon:

1. Fully shared octet of electrons in its compounds
2. Can form strong single, double and triple bonds to itself
3. Can for four covalent bonds

Empirical, molecular and structural formula

Empirical Formula:

1. Simplest formula that shows the ratio of each kind of atoms in a molecule.

• Example: CH₄ for all alkanes

Molecular formula:

1. Shows the actual number of each kind of atoms in a molecule

• Example: C₂H₈ for ethane

Structural formula:

1. Shows how each atom is connected to another atom in a molecule

Condensed structural formula:

1. Shows the arrangement of atoms in an unambiguous manner, without showing the bonds

• Example: CH₄ for methane

Displayed formula / Full structural formula:

1. Shows all the bonds and relative placing of all the atoms

• Example: O=C=O for Carbon-Dioxide

Stereochemical / 3D structural formula:

1. Shows the shape of three-dimensional molecule;

• Solid wedges indicate bonds coming out of plane;

• Dashed wedges indicate bonds going into the plane;

• Solid lines indicate bonds on the plane;

• Example: Sorry I can’t draw.

Skeletal formula:

1. Bare minimum, omitting all carbon and hydrogen atoms;

• Useful for larger and more complicated molecules;

• Example: Sorry I can’t draw.

Functional groups and naming of organic compounds

Functional group:

1. A group of atoms within a molecule that has a characteristic chemical behaviour.
2. Behaves in nearly the same way in every molecule it is part of.

Example: C=C in ethene and cholesterol reacts similarly towards bromine

All molecules containing the same functional group can be considered as members of same homologous series, have similar properties.

Homologous series; Functional groups	Prefix	Suffix	Example
Alkenes; -C=C-		-ene	Ethene
Arenes; C6H5-	phenyl	Benzene	Phenylethene/Methylbenzene
Halogenoalkanes / Halogenoarenes; -F -Cl -Br -I	Fluoro Chloro Bromo Iodo		Fluoroethane
Alcohols; -OH	Hydroxy	-ol	Ethanol
Aldehydes; -CHO		-al	Ethanal
Ketones; -CO-	Oxo	-one	Oxobutanal/Propanone
Carboxylic Acids; -COOH		-oic acid	Ethanoic acid
Esters; -COOR		-oate	Methylethanoate
Acid Halides; -CXO		-oyl halide	Ethanoyl fluoride
Amines; -NH2	Amino	-amine	Ethylamine
Amides; -CONH2		-amide	Ethanamide
Nitriles; -CN	Cyano	-nitrile	Ethannitrile

Characteristics of Organic Reactions

Types of Organic Reactions

Addition:

1. 2 substances react together to form a single product
2. An unsaturated functional group is usually involved

Elimination:

1. A small molecule (from atoms attached to neighbouring carbons) such as H₂O, or HBr, or HCl is eliminated, from a larger one

Substitution:

1. One atom or group of atoms substituted by another
2. Always at least 2 products

Oxidation:

1. Oxygen atoms added to molecule OR
2. Hydrogen atoms removed from molecule
3. In balancing equations for organic redox reactions, [O] denotes oxidation by oxidizing agent (+ O₂ if oxidized by oxygen in the air)

Reduction:

1. Hydrogen atoms added to molecule AND / OR
2. Oxygen atoms removed from molecule
3. In balancing equations for organic redox reactions, [H] denotes reduction by reducing agent (+H₂ if H₂ is used)

Condensation:

1. 2 molecules come together to form a bigger molecule, with the elimination of a small molecule such as water

Hydrolysis:

1. Molecule is split into 2 by action of water, often catalysed by dil. Acids / dil. Alkalis.

Mechanisms of Organic Reactions

Mechanism:

• Description of a successful collision between the reactants and the electron rearrangements that happen as reactants are changed to product.
• A sequence o steps by which the reaction takes place.

Most mechanisms are a series of labelled diagrams showing representation of electron reorganisation of the particles:

1. Just before the successful collision
2. Identification of any intermediate species that may be formed during the collision
3. After the collisions in the reaction

Arrow notation:

1. Full head: movement of electron PAIR (Example: Heterolytic fission / heterolysis)
2. Partial head: movement of a SINGLE electron (Example: Homolytic fission / homolysis)

Organic Reagents

Classified by kind of reactive sites attacked:

1. Electrophile

• Attacks positions of high electron density
• Cation OR
• Polar bond (partial positive charge)
• Examples: NO₂⁺, HBR

1. Nucleophile

• Attacks positions of electron deficiency
• Anion OR
• Molecule with lone pair on one of its atoms
• Examples: OH^-, NH₃

Classified by type of reaction caused:

1. Oxidising Agent

• Example: Acidified KMnO₄

1. Reducing Agent

• Example: H₂ with Nickel catalyst

Shapes of Organic Molecules: pi and sigma bonds

Bonding in Organic Molecules

• Carbon can form 4 covalent bonds
• Carbon atoms can be arranged in straight / branched chains, rings
• C-C bonds and C-H bonds stronger than bonds with most other elements due to small atomic size
• C-H bond is almost non-polar due to very small diff in electronegativities. Hydrocarbon part is unreactive and stable.

sp³ hybridisation

1. Excitation of one of the electrons in the 2s orbital to vacant 2p orbital
2. Hybridisation of 2s and 3 2p orbitals to form 4 sp³ orbitals (equivalent shape, size and energy)
3. New orbitals spatially orientated in tetrahedral arrangement (maximum separation of 4 orbitals to minimize repulsion between electrons; 109.5^o)

sp² hybridisation

1. Excitation of one of the electrons in the 2s orbital to vacant 2p orbital
2. Hybridisation of 2s and 2 2p orbitals to form 3 sp² orbitals (equivalent shape, size and energy)
3. New orbitals spatially orientated in trigonal planar arrangement (maximum separation of 3 orbitals to minimize repulsion between electrons; 120^o)

sp hybridisation

1. Excitation of one of the electrons in the 2s orbital to vacant 2p orbital
2. Hybridisation of 2s and 1 2p orbital to form 2 sp orbitals (equivalent shape, size and energy)
3. New orbitals spatially orientated in linear arrangement (maximum separation of 2 orbitals to minimize repulsion between electrons; 180^o)

Isomerism: Structural, geometrical, optical

Isomerism: Phenomenon where certain compounds possessing same molecular formula exist in different forms because they have different arrangement of atoms.

Structural Isomerism: Atoms linked together in different ways

1. Chain isomerism: Different carbon skeleton
2. Positional isomerism: Functional group(s) at different positions
3. Functional group isomerism: Different functional groups

Stereoisomerism: Atoms have different spatial arrangements

1. Geometric isomerism (cis-trans isomerism): Caused by restricted rotation of C=C OR ring

• Cis: Heavy groups on same side of C=C
• Trans: Heavy groups on opposite sides of C=C
• Same chemical properties
• Different physical properties

1. Optical isomerism: Non-superimposable mirror images of each other

• Optically active: rotate plane-polarized light
• One chiral carbon: Carbon with 4 different groups attached
• No plane of symmetry
• Same physical properties BUT
• Rotate light in opposite directions
• Same chemical properties EXCEPT
• Towards other optically active compounds

Conclusion

Organic chemistry is important as it involves many molecules responsible for life.

Carbon can form many compounds because of ability to form strong bonds with other carbon atoms and other elements.

Organic reactions are very dependent on conditions of the experiment.

• May give rise to different organic compounds when treated with same reagent under different conditions
• Requires understanding of mechanism of organic reactions

Organic chemistry can be understood by: nature of functional groups & movement of electrons during reaction.