Trends in Chemical Reactivity of Group 13 Elements

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Unlike boron, the other members of the group are metallic and the electropositive character increases as we go down the group. All these elements in their compounds exhibit the oxidation state of + 3 and +1. The important chemical properties of the boron family are discussed below:

1. Hydrides

None of the group 13 elements reacts directly with hydrogen. However, a number of hydrides of these elements have been prepared by indirect methods. The boron hydrides are sometimes called boranes. The boranes can be classified with two series:

(a) B_nH_n+4 called nidoboranes (b) B_nH_n+6 called arachnoboranes

The simplest borane is diborane, B₂H₆ (BH₃ being unstable has not been isolated). It is prepared on the industrial scale by the reaction of lithium hydride with BF₃.

Preparation of Diborane: In laboratory, it can be obtained by following methods:

(i) By the reaction of iodine with sodium borohydride in a high boiling solvent.

(ii) By reduction of BCl₃ with LiAlH₄

Some important characteristics of boranes are given below:

i) Lower boranes are colourless gases while higher boranes are volatile liquids or solids.

ii) They undergo spontaneous combustion in air due to strong affinity of boron for oxygen.

iii) Boranes react with alkali metal hydrides in diethyl ether to form borohydride complexes.

Alkali borohydrides are used as reducing agents.

(iv) Diborane reacts with ammonia to give borazine at 450 K.

Borazine has a cyclic structure similar to benzene and thus, it is called inorganic benzene.

In contrast to hydrides of boron, the other elements of this group form only a few stable hydrides. The thermal stability decreases as we move down the group. AlH₃ is a colourless solid polymerised via Al - H - Al bridging units. These hydrides are weak Lewis acids and readily form adducts with strong Lewis base (B:) to give compounds of the type MH₃ (M = Al or Ga). In addition, they also form complex (tetrahydrido anions, [MH₄]^-. The most important tetrahydrido compound is a complex hydride of aluminium, Li[AlH₄] is known. It is prepared by reaction of AlCl₃ with excess of LiH in ether.

Lithium tetrahydrido-aluminate (III), LiA1H₄, is a white crystalline solid and soluble in diethyl ether is a versatile reducing agent used in organic synthesis. Gallium is also known to form a complex hydride, Li[GaH₄].

2. Oxides and hydroxides

Almost all the elements of group 13 form oxides and hydroxides of the composition M₂O₃ and M(OH)₃ respectively. The nature of these oxides / hydroxides change from weakly acidic to amphoteric and amphoteric to basic in moving down the group from B to Tl.

Reason

As one moves down the group, the atomic size of elements goes on increasing whereas the ionization energy decreases. As a result, the strength of M-O bond goes on decreasing accounting for the increase in basic character down the group or conversely explains the decrease in acidic character.

Aluminium and gallium hydroxides show amphoteric behavior.

In contrast to Tl(OH)₃, which is insoluble in water, T1(OH) is soluble and is a strong base. Many of the T1(I) compounds are similar to the corresponding alkali metal compounds.

3. Halides

Al, Ga, In and Tl react with halogens to give binary halides. All the halides of group 13 elements are known except Tl(III) iodide. The fluorides are ionic and have high melting points. The chlorides, bromides and iodides are essentially covalent compounds with low melting points.

The boron trihalides are covalent in nature and exist as monomeric molecules having planar triangular geometry. In these halides boron assumes sp² hybrid state and the three B-X bonds are formed by axial overlap of sp² hybrid orbital of boron and p-orbital of halogen.

fig 8.1 - Structure of boron trihalides

Since there are only six electrons in the valence shell of boron atom in boron trihalides, therefore, they have a great tendency to accept two more electrons in order to acquire a stable octet.

Dot diagram

Hence boron trihalides act as Lewis acids. They rapidly combine with electron donors such as NH₃, F^- ion, amines, ether, sulphides or phosphorus.

The other halides of Group 13 elements have halogen bridged dimeric structures as shown below for aluminium (III) chloride, which exists as Al₂Cl₆. In the dimeric structure each aluminium atom accepts a lone pair of electrons from the chlorine atom bonded to the other aluminium atom. In doing so each aluminium atom completes its octet.

fig 8.2 - Dimeric structure of aluminium chloride

Note:

Boron halides do not form dimers because the size of boron is so small that it is unable to coordinate four large-sized halide ions.

The trihalides of group 13 elements are strong Lewis acids. As a consequence of its strong Lewis acid character, BF₃ is used as a catalyst in several industrial processes. Anhydrous aluminium chloride is used as a catalyst in several organic reactions (Friedel - Craft's reaction).