Saturated molecule is a molecule in which each carbon atom has the maximum number of single bonds possible (four) and unsaturated molecule is a molecule that contains one or more carbon–carbon multiple bonds.

The steps for naming an alkene or an alkyne are:

• STEP 1: Name the parent compound. Find the longest chain containing the double or triple bond, and name the parent compound by adding the suffix -ene or -yne to the name for the main chain.

  • if there are more than one double or triple bonds, the number of multiple bonds is indicated using a numerical prefix( diene= two double bonds, triene= three double bonds and so forth).

• STEP 2: Number the carbon atoms in the main chain so that those with multiple bonds have the lowest index numbers possible.

  • Thus, begin numbering at the end nearer the multiple bond (Examples 1 and 3). If the multiple bond is an equal distance from both ends, begin numbering at the end nearer the first branch point.

• STEP 3: Write the full name. Assign numbers to the branching substituents, and list the substituents alphabetically. Use commas to separate numbers and hyphens to separate words from numbers.

  • Indicate the position of the multiple bond in the chain by giving the number of the first multiple-bonded carbon. If more than one double bond is present, identify the position of each and use the appropriate name ending.

Unlike in alkanes, where free rotation around the C¬C single bond occurs, there is no rotation around a double bond, and the molecules are more rigid.

• However, their restricted freedom of rotation makes a new kind of isomerism possible for alkenes. As a consequence of their rigid nature, alkenes possess ends and sides.

Cis–trans isomer are alkenes that have the same connections between atoms but differ in their three-dimensional structures because of the way those groups attach to different sides of the double bond.

Properties of Alkenes and Alkynes:

• Nonpolar; insoluble in water; soluble in nonpolar organic solvents; less dense than water • Flammable; nontoxic

• Alkenes display cis–trans isomerism when each double-bond carbon atom has different substituents

• Cis-trans isomers can have different physical and biological properties.

• Multiple bonds are chemically reactive.

Four particularly important kinds of organic reactions involving alkynes and alkenes: additions, eliminations, substitutions, and rearrangements.

• Addition reaction: A general reaction type in which a substance X¬Y adds to the multiple bond of an unsaturated reactant to yield a saturated product that has only single bonds.

• Elimination reaction :A general reaction type in which a saturated reactant yields an unsaturated product by losing groups from two adjacent atoms.

• Substitution reaction: A general reaction type in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms.

• Rearrangement reaction: A general reaction type in which a molecule undergoes bond reorganization to yield an isomer.

Hydrogenation is the addition of H2 to a multiple bond to give a saturated product.

Halogenation (usually for alkene) is the addition of Cl2 or Br2 to a multiple bond to give a dihalide product.

Hydrohalogenation is the addition of HCl or HBr to a multiple bond to give an alkyl halide product.

Markovnikov’s rule states that in the addition of HX to an alkene, the major product arises from the H attaching to the double-bond carbon that has the larger number of H atoms directly attached to it and the X attaching to the carbon that has the smaller number of H atoms attached.

Hydration is the addition of water to a multiple bond to give an alcohol product.

Polymer is a large molecule formed by the repetitive bonding together of many smaller molecules (or monomers) and a monomer is a small molecule that is used to prepare a polymer.

• Many simple alkenes undergo polymerization reactions when treated with the proper catalyst. Ethene yields polyethene upon polymerization, propene yields polypropene, and styrene yields polystyrene.

  • The polymer product might have anywhere from a few hundred to a few thousand monomer units incorporated into a long, repeating chain.

• The reaction begins by addition of a species called an initiator to an alkene; this results in the breaking of one of the bonds making up the double bond.

  • A reactive intermediate that contains an unpaired electron (known as a radical) is formed in this step, and it is this reactive intermediate that adds to a second alkene molecule.

• This produces another reactive intermediate, which adds to a third alkene molecule, and so on. Because the result is continuous addition of one monomer after another to the end of the growing polymer chain, polymers formed in this way are chain-growth polymers.

Aromatic is the class of compounds containing benzene-like rings.

Rather than being held between specific pairs of atoms, the double-bond electrons are instead free to move over the entire ring. Each carbon–carbon bond is thus intermediate between a single bond and a double bond.

• This is known as resonance, where the true structure of a molecule is an average among two or more possible conventional structures.

• Resonance allows the electrons in the double bonds to be delocalized over the entire molecule, thus lowering the reactivity of the double bonds. It is important to note that no atoms move between resonance structures, only pairs of electrons .

When a benzene has more than one substituent present, the positions of those substituents are indicated by numbers, just as in naming cycloalkanes.

Disubstituted benzenes (and only disubstituted benzenes) are unique in that the relational descriptors o- (ortho), m- (meta), and p- (para) may be used in place of 1,2-, 1,3-, and 1,4-, respectively. The terms ortho-, meta-, or para- (or their single-letter equivalents) are then used as prefixes.

Reactions involving aromatic compounds are:

• Nitration is the substitution of a nitro group (¬NO2) for a hydrogen on an aromatic ring.

• Halogenation is the substitution of a halogen group (¬X) for a hydrogen on an aromatic ring.

• Sulfonation is the substitution of a sulfonic acid group (¬SO3H) for a hydrogen on an aromatic ring.