JEE Chemistry Organic Chemistry Some Basic Principles And Techniques Notes Set 23

Dipole Moment

Dipole moment is the product of the separated charge and the distance between the charges, \( \mu = e \times d \). It is a vector that points from the positive to the negative end, and its unit is the debye, \( 1\,D = 10^{-18} \) esu cm.
A molecule is non-polar only if the bond moments cancel by symmetry, as in \( \ce{CCl4} \), \( \ce{CO2} \), \( \ce{BF3} \), \( \ce{SF6} \) and \( \ce{XeF4} \). Lone pairs on the central atom, as in \( \ce{H2O} \), \( \ce{NH3} \) and \( \ce{CHCl3} \), destroy the symmetry and give a net moment.
In methyl halides the bond length and electronegativity effects oppose each other, so the molecular moments run \( \ce{CH3Cl} > \ce{CH3F} > \ce{CH3Br} > \ce{CH3I} \). Among chloromethanes the order is \( \ce{CH3Cl} > \ce{CH2Cl2} > \ce{CHCl3} > \ce{CCl4} \) (zero).

Dipole Moment of Alkenes and Disubstituted Benzenes

The trans isomer of an alkene has zero moment only when the two groups on each carbon are the same, or are linear in their moments. A cis isomer such as cis-1,2-dichloroethene (1.9 D) is polar, whereas the trans isomer is zero.
For a disubstituted benzene the resultant is \( \mu = \sqrt{\mu_1^2 + \mu_2^2 + 2\mu_1\mu_2\cos\theta} \). If both groups are donating or both withdrawing, the order is ortho > meta > para, and the para isomer is zero when the groups are identical and have linear moments (for example p-dichlorobenzene or p-dicyanobenzene).
If one group donates and the other withdraws, the moments add in the para position and the order reverses: para > meta > ortho.

Intermolecular Forces

Dipole-dipole attraction acts between polar molecules. Hydrogen bonding is the strongest form and needs H bonded to F, O or N; the strength falls in the order H-F...H > H-O...H > H-N...H.
Intermolecular hydrogen bonding associates molecules (carboxylic acid dimers, p-nitrophenol, water). Intramolecular hydrogen bonding (chelation) occurs between ortho substituents, as in o-nitrophenol, and it reduces association, so these isomers are more volatile and less soluble in water.
London (van der Waals) forces act in all molecules and increase with surface area and polarisability. This is why \( \ce{CCl4} \) (non-polar) boils higher than \( \ce{CHCl3} \).

Boiling Point

A liquid boils when its vapour pressure equals the external pressure, so stronger intermolecular forces mean a higher boiling point.
Alcohols boil higher than ethers and alkanes of similar mass because of hydrogen bonding. Primary and secondary amines also hydrogen bond, but more weakly than alcohols, since N is less electronegative than O. Tertiary amines have no N-H and boil lower.
Within a homologous series the boiling point rises with molecular mass. Among isomers the straight chain boils higher than the branched one. Haloalkanes boil higher as the halogen gets larger: RI > RBr > RCl > RF.
Ortho isomers capable of chelation boil lower than meta and para isomers; for example o-nitrophenol is steam volatile whereas p-nitrophenol is not.

Melting Point

Melting point depends on the strength of the forces and especially on how well the molecules pack in the crystal. Symmetrical molecules have higher melting points: p-dichlorobenzene melts higher than the ortho and meta isomers, and neopentane-type spherical molecules such as 2-methylpropan-2-ol melt higher than straight chain isomers.
Para isomers usually melt higher than ortho and meta isomers. Compounds that hydrogen bond strongly (dicarboxylic acids, diols, hydroquinone) melt higher, and fused ring hydrocarbons melt higher as the number of rings increases (linear fusion more than angular).

Solubility in Water

A compound dissolves in water when it can form hydrogen bonds with water that compensate for breaking its own intermolecular attractions. Lower alcohols, acids, amines, aldehydes and ketones are soluble; the large hydrocarbon part reduces solubility as the chain grows. Branched isomers are more soluble than straight chain isomers.
Compounds that chelate (o-nitrophenol, salicylaldehyde) are less soluble than their para isomers, which hydrogen bond with water. Higher melting, better-packed isomers tend to be less soluble (the lattice must be broken). Alcohol > ether > alkene > alkane for similar mass.

Separation by Acid-Base Extraction

A mixture of organic compounds in an organic solvent is separated by the different acidic or basic character of its components.
Strong acids such as carboxylic acids dissolve in aqueous \( \ce{NaHCO3} \) with evolution of \( \ce{CO2} \). Phenols are too weakly acidic for \( \ce{NaHCO3} \) but dissolve in aqueous \( \ce{NaOH} \). Amines dissolve in aqueous HCl by forming water soluble ammonium salts. Neutral compounds such as hydrocarbons, ethers and aldehydes remain in the organic layer.
Each extracted component is recovered by neutralising its aqueous extract (acidifying the carboxylate or phenoxide solution, basifying the ammonium solution).

Lassaigne's Test and Distinguishing Tests

In Lassaigne's test the compound is fused with sodium so that N, S and halogens are converted into ionic NaCN, Na2S and NaX, which are then tested in the aqueous extract. Nitrogen gives Prussian blue; sulphur gives violet colour with sodium nitroprusside; N and S together give blood red ferric thiocyanate; halogens give precipitates with silver nitrate.
Aromatic primary amines are distinguished from aliphatic ones by the azo dye test: diazotisation at 0 to 5 degrees C followed by coupling with beta-naphthol gives an orange-red dye, whereas aliphatic amines evolve nitrogen. Both give the carbylamine test.