Revision: Halogen Derivatives Chemistry HSC Science (General) 12th Standard Board Exam Maharashtra State Board

The relationship between a chiral molecule and its mirror image is similar to the relationship between left and right hands. Therefore it is called handedness or chirality

A light having oscillations only in one plane perpendicular to direction of propagation of light is known as plane polarized light.

Active metals like sodium, magnesium cadmium readily combine with alkyl chlorides, bromides and iodides to form compounds containing carbon-metal bonds. These are known as organometallic compounds.

Halogen derivatives in which halogen is bonded to an sp³ hybridised carbon of an alkyl group are called haloalkanes or alkyl halides.

Isomers having the same bond connectivity but different spatial arrangement are called stereoisomers.

Isomerism arising due to different spatial arrangement around a chiral carbon atom is called optical isomerism.

Two non-superimposable mirror image structures are called enantiomers.

A substance which rotates plane polarized light to the right (clockwise) is called dextrorotatory compound.

Halides in which halogen is attached to an sp³ carbon adjacent to a double bond are called allylic halides.

Halides in which halogen is attached to an sp³ carbon next to an aromatic ring are called benzylic halides.

Halides in which halogen is attached to an sp² hybridised carbon of a double bond are called vinylic halides.

The attractive forces between molecules due to temporary dipoles are called van der Waals forces.

A reaction in which one atom or group is replaced by another atom or group is called a substitution reaction.

The reaction of alkyl halide with magnesium in dry ether to form alkyl magnesium halide is called Grignard reaction.

The compound of the type R–Mg–X formed by reaction of alkyl halide with magnesium is called Grignard reagent.

The reaction of alkyl halides with sodium metal in dry ether forming higher alkanes is called Wurtz reaction.

Halogen derivatives in which halogen is directly attached to an aromatic ring are called haloarenes or aryl halides.

Compounds having the same molecular formula but different physical or chemical properties are called isomers.

A substance which rotates plane polarized light to the left (anticlockwise) is called levorotatory compound.

Light vibrating in only one plane perpendicular to the direction of propagation is called plane polarized light.

A substitution reaction in which a nucleophile replaces a leaving group is called a nucleophilic substitution reaction.

A reaction in which the rate depends on the concentration of two reacting species is called a bimolecular reaction.

The inversion of configuration observed in SN₂ reaction due to backside attack is called Walden inversion.

Formation of equal amounts of two enantiomers in an SN₁ reaction is called racemization.

A chemical species that donates an electron pair to form a bond with carbon is called nucleophile.

A solvent capable of hydrogen bonding and stabilizing ions is called polar protic solvent.

A solvent that cannot donate hydrogen for hydrogen bonding and does not strongly solvate anions is called polar aprotic solvent.

Removal of hydrogen halide (HX) from an alkyl halide to form alkene is called dehydrohalogenation reaction.

The rule stating that in elimination reaction the major product is the more substituted alkene is called Saytzeff (Zaitsev) rule.

1. Dichloromethane (CH₂Cl₂) is a volatile solvent used in aerosols and industrial processes, but overexposure can cause dizziness, nausea, weakness and serious eye damage.
2. Chloroform (CHCl₃) is used as a solvent and in the preparation of refrigerants, but prolonged exposure depresses the central nervous system and may damage the liver.
3. Carbon tetrachloride (CCl₄) is used as a solvent and cleaning agent, but it is highly toxic to the liver and can affect the central nervous system; it is also suspected to be carcinogenic.
4. Iodoform (CHI₃) is a yellow crystalline compound formerly used as an antiseptic, but it can cause irritation to skin, eyes and respiratory system.
5. Freons (chlorofluorocarbons, CFCs) are widely used as refrigerants and propellants, but they are responsible for depletion of the ozone layer in the atmosphere.
6. Ultraviolet radiation breaks CFC molecules to release chlorine atoms, and a single chlorine atom can destroy thousands of ozone molecules, leading to thinning of the ozone layer.
7. DDT is an effective insecticide used to control malaria and typhus, but it is a persistent pollutant that accumulates in fatty tissues, harms wildlife and has serious long-term environmental and health effects.

1. Optical isomerism arises due to different spatial arrangement of atoms around a chiral carbon atom, even though the molecular formula and bonding are the same.
2. A carbon atom attached to four different atoms or groups makes the molecule chiral and capable of showing optical activity.
3. Chiral molecules are not superimposable on their mirror images, just like left and right hands.
4. The two mirror-image forms of a chiral compound are called enantiomers and they differ only in the direction in which they rotate plane polarized light.
5. Optical activity is the property by which certain compounds rotate the plane of plane polarized light passing through them.
6. If the plane of polarized light rotates to the right, the compound is dextrorotatory (+), and if it rotates to the left, it is levorotatory (−).
7. Enantiomers have identical physical properties such as melting point and boiling point, but they differ in the direction of optical rotation and in their behavior towards other chiral substances.
8. An equimolar mixture of two enantiomers does not rotate plane polarized light because the rotations cancel each other, and such a mixture is optically inactive.