Fundamental Principles of Organic Chemistry

Principle: nitrogen in the organic compound is converted by fusion with sodium into ionic sodium cyanide, which is then detected as Prussian blue.

Procedure

1. Preparation of sodium extract: a small piece of dry sodium metal is heated in a fusion tube until it melts, and a little of the organic compound is added. The tube is heated strongly to red hot and plunged into distilled water in a china dish. It is ground, boiled and filtered. The filtrate is the Lassaigne's (sodium fusion) extract.

$\mathrm{Na + C + N \xrightarrow{\Delta} NaCN}$

2. Test: to a portion of the extract add freshly prepared FeSO$_4$ solution and boil. Sodium hexacyanoferrate(II) forms:

$\mathrm{6NaCN + FeSO_4 \rightarrow Na_4[Fe(CN)_6] + Na_2SO_4}$

3. Add a few drops of FeCl$_3$ solution (or warm and acidify with dilute H$_2$SO$_4$/HCl, which dissolves the iron(II) hydroxide). Hexacyanoferrate(II) reacts with Fe$^{3+}$ to give

$\mathrm{3Na_4[Fe(CN)_6] + 4FeCl_3 \rightarrow Fe_4[Fe(CN)_6]_3 + 12NaCl}$

Result: a Prussian blue colour or precipitate confirms nitrogen. (If sulphur is also present, a blood-red colour of Fe(SCN)$^{2+}$ may form instead.)

(i) Inductive effect: the permanent displacement of the shared electrons of a σ bond along a carbon chain, caused by an electronegative atom or group attached to it. Example: in $\mathrm{Cl\!\to\!CH_2\!-\!CH_3}$, Cl pulls the electrons toward itself.

(ii) Electrophile: an electron-deficient species that can accept an electron pair from another species to form a covalent bond, e.g. $\mathrm{H^+}$, $\mathrm{NO_2^+}$, $\mathrm{AlCl_3}$.

Inductive effect: the permanent polarisation (displacement) of $\sigma$-electrons along a saturated carbon chain, caused by the presence of an electron-withdrawing ($-I$) or electron-donating ($+I$) atom or group.

Use: it explains the relative acid strengths of carboxylic acids. For example, $\mathrm{ClCH_2COOH}$ is a stronger acid than $\mathrm{CH_3COOH}$, because the $-I$ effect of chlorine stabilises the carboxylate ion. It likewise explains the basic strength of amines and the stability of carbocations.

A homologous series is a series of organic compounds having the same functional group, in which each successive member differs from the previous one by a –CH₂– unit (i.e., by a molecular mass of 14 amu), and which can be represented by a common general formula. All members show similar chemical properties and a gradual change in physical properties.

Mass difference. Successive members differ by one $-\text{CH}_2-$ unit:

$\text{Mass difference} = 12 + 2(1) = 14\ \text{u}$

Molecular mass of X. The series is CH$_3$OH, CH$_3$CH$_2$OH, X, CH$_3$CH$_2$CH$_2$CH$_2$OH, so X is the third member, propan-1-ol, $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$ ($\text{C}_3\text{H}_8\text{O}$).

$M_r = 3(12) + 8(1) + 16 = 60\ \text{u}$

Check: ethanol is 46 u, so $46 + 14 = 60$ u.

The "fourth member" of the homologous series of alcohols is butan-1-ol (C₄H₉OH).

Highest boiling point: Boiling point increases down a homologous series because molecular mass and molecular size increase, which strengthens the van der Waals (London dispersion) forces between molecules. Among the first four alcohols (methanol → butanol), butanol has the largest molecular mass and longest carbon chain, so it has the strongest intermolecular forces and hence the highest boiling point.

Least solubility in water: Solubility in water is due to the polar –OH group, which forms hydrogen bonds with water. As the hydrocarbon (alkyl) chain lengthens, the non-polar part grows and dominates over the small polar –OH group. The alkyl part is hydrophobic and does not interact with water; it actually disrupts the water–water hydrogen bonding. So in butanol the non-polar character outweighs the polar character, making it the least soluble of the four.

In short: The same factor — the increasing size of the non-polar alkyl group — raises the boiling point (stronger van der Waals forces) but lowers water solubility (weaker interaction with water relative to the hydrophobic chain).