Boron group.pdf

BORON

I) HYDRIDES

II) CARBORANES

III) NITRIDES

IV) Halides

Dr Sonika Batra

GD Goenka

2

Groups of Elements in the Periodic Table

Eight Groups (the 7 groups of representative elements and the group of noble gases):

I. Alkali metals: (H), Li, Na, K, Rb, Cs II. Alkali earth metals: Be, Al, Ca, Sr, Ba, Ra III.Boron family: B, Al, Ga, In, Tl

IV. Carbon family: C, Si, Ge, Sn, Pb V. Nitrogen family: N, P, As, Sb, Bi VI. Chalcogens O, S, Se, Te, Po

VII.Halogens F, Cl, Br, I, At

Extraction of Boron

•

Amorphous boron is obtained by reducing B

O

with Mg or Na at a

high temperature

DiBorane -

B

2

H

6

Diborane is the chemical compound consisting of boron and hydrogen

with the formula B

H

.

It is a colorless and highly unstable gas at room temperature with a

sweet odor.

Diborane mixes well with air, easily forming explosive mixtures.

Production and synthesis

• Extensive studies of diborane have led to the development of multiple syntheses.

• By reduction of Boric acid with Hydrogen and Al B₂O₃ + 3H₂ + 2Al B₂H₆ + Al₂O₃

• The industrial synthesis of diborane involves the reduction of BF₃ by sodium hydride, lithium hydride or lithium aluminium hydride

Two laboratory methods start from boron trichloride with lithium aluminium hydride or from boron trifluoride ether solution with sodium borohydride.

8 BF₃ + 6 LiH → B₂H₆ + 6 LiBF₄

Similarly, oxidation of borohydride salts has been demonstrated and remains convenient for small scale preparations. For example, using iodine :

• 2 NaBH₄ + I₂ → 2 NaI + B₂H₆ + H₂

Diborane- Structure

• The bonds between boron and the terminal hydrogen atoms are conventional center, 2-electron covalent bonds.

• Having used two electrons in bonding to the terminal hydrogen atoms, each boron has one valence electron remaining for additional bonding. The bridging hydrogen atoms provide one electron each. Thus the B₂H₂ ring is held together by four electrons, an example of 3-center 2-electron bonding.

• This type of bond is sometimes called a 'banana bond'.

• The lengths of the B-H bridge bonds and the B-H terminal bonds are 1.33 and 1.19 Å resp

• The B-H bridge bonds being relatively weaker.

• The structure is isoelectronic with C₂H₆ 2+_{, which would arise from the diprotonation of the}

Structure of Boranes:

There are three important structures of boranes (closo-, nido- and arachno-) In this structures the boron atoms are occupying the corners of a polyhedron in which the boron atoms can be bound together. These structures are so called cage structures.

a) closo – B_nH_n2- _{eg. B}

6H62- is closo-type and the 6 B’s lie on the corners of a octahedron

b) nido – B_nH_n+4 eg. B₅H₉ is nido-type and the 5 B’s lie on the corners of a square pyramid where one corner is removed

c) arachno – B_nH_n+6 eg. B₄H₁₀ is arachno-type and the 4 B’s lie on the corners of an octahedron where two corners are removed

d) hypho- B_nH_n+8 , net like open poyhedra with B atoms occupying n corners of polyhedra e. g. B₈H₁₆

Wade Rules or

polyhedral skeletal electron pair theory

A method to determine the geometry of boranes are the Wade rules. The Wade rules are a

correlation between the number of electrons , the formula and the shape of the molecule. To use this method, the total number of valence electrons that are forming the bonds must be determined (n = Number of boron atoms).

The general methodology to be followed when applying Wade’s rules is as follows: 1. Determine the total number of valence electrons from the chemical formula, i.e., 3 electrons per B, and 1 electron per H.

2. Subtract 2 electrons for each B-H unit (or C-H in a carborane).(this is actually equal to number of B atoms present)

3. A negative charge contributes electrons

Type Formula Skeletal Electron_Pairs

Closo _[B

nHn]2- n + 1

Nido BnHn+4 _{n + 2}

Arachno _{BnHn+6 n + 3}

Hypho _{BnHn+8 n + 4}

Conjucto _B

Example: [B

H

]

•

Number of valence electrons = 6(3) + 6(1) + 2 = 26

•

Number of electrons for each BH unit = 2X6 = 12

•

SEP = ½ (26-12)= 7

•

SEP= 6+1= n+1

Total number of valence electrons = (5 x B) + (11 x H) = (5 x 3) +

(11 x 1) = 26

1.Number of electrons for each B-H unit = (5 x 2) = 10

2.Number of skeletal electrons = 26 – 10 = 16

3.Number SEP = 16/2 = 8

4.Structure of

n

+3, with n=5, therefore B

H

is

an

arachno

based upon a pentagonal bipyramid with two

•

Total number of valence electrons = (5 x B) + (9 x H) = (5 x 3) + (9 x 1)

= 24

•

Number of electrons for each B-H unit = (5 x 2) = 10

•

Number of skeletal electrons = 24 – 10 = 14

•

Number SEP = 14/2 = 7

Properties of Diborane

• It is a colourless gas (b.p.,-92.6 and m.p.,-164.9 C) with a foul smell and is extremely toxic.

(2) Thermal Stability.

• It is stable only at low temperatures. On heating at temperatures above 100 C in sealed vessels, it decomposes to form a number of higher

hydrides

(3) Action of Oxygen.

4) Hydrolysis.

• It is immediately hydrolyzed by water to give hydrogen and therefore acts as a reducing agent.

It reacts with methanol in a similar reaction

(5) Action with Halogens.

(6) Action with Halogens Acids (HX).

Reacts to give halodiboranes

(7) Action with Ammonia.

It reacts with ammonia to give different products under different reaction conditions.

Although often compared with benzene, borazine is far more reactive. With hydrogen chloride it forms an adduct, whereas benzene is unreactive toward HCl.

Polyborazylene

• Addition reaction of borazine with hydrogen chloride B₃N₃H₆ + 3 HCl → B₃N₃H₉Cl₃

Reaction with water

B₃N₃H₆+ 9H₂O → 3 NH₃ + 3 H₃BO₃ + 3H₂

• Borazines undergo nucleophilic attack at boron and electrophilic attack at nitrogen.

• Heating borazine at 70 °C expels hydrogen with formation of

a borazinylpolymer or polyborazylene, in which the monomer units are coupled in a

(8) Action with Alkali.

Diborane dissolves in strong alkaline solutions (KOH) giving metaborates and H gas is evolved.

(9) Action with Metals.

Diborane reacts slowly (over a few days) with electro-positive metals such as Na, K Ca or their amalgams.

The reaction becomes fast in ethers (used as solvent).

B₃ H₈- _{was the first polyborate anion prepared by this reaction. This anion is now}

(10) Action with Amines.

It reacts with tertiary amines to form amine-borane complex.

These complexes can serve as a source of diborane in chemical reactions since thy can be stored or shipped safely.

(11) Action with Hydrides.

It reacts with sodium hydride to form a complex hydride.

(12) Action with CO.

(13) Action with Boron halides.

(14) Action with Ethers and Thioethers.

It combines with ether or thioether to form addition compounds.

(15) Action with Pyridine

(16) Action with Alcohols.

Addition to alkenes (Hydroboration

Diborane adds to alkenes and alkynes in ether solvents at room temperature to form alkyboranes. This reaction is known as HYDROBORATION. This reaction leads to the formation of organoboranes which yield to a variety of synthetic organic chemistry. Hydroboration is regiospecific, the boron showing preferential attachment to the least substituted carbon atom (anti-Markonikov)

(i) On refluxing it with anhydrous carboxylic acid yields the alkane corresponding to the initial alkene.

Applications/Uses:

•

Diborane has been suggested as a rocket propellant ( but could not be

used because of incomplete combustion to B

O

and instead BO

polymer was formed which partly blocked the nozzels of the rocket.

•

Used as a rubber vulcaniser,

•

as a catalyst for hydrocarbon polymerisation,

•

and as a doping agent for the production of semiconductors.

Carboranes

Carboranes are borane clusters having carbon atoms replacing the framework boron atoms. They also has been classified based on the structure like closo, nido, arachno and hypho.

Structure is determined by using Wade’s rule as used for Boranes. In this case , the contribution of each CH unit towards skeletal bonding is

considered to be 2. The total number of skeletal electron pairs (SEP) cab be calculated as:

SEP= ½( total number of valence electrons of B, C and H + Negative charge – Total number of

For example C

B

H

SEP= ½( 4X2 + 3X4 + 1X6 + - 2X4 – 2X2) = 7

So this is 6+1= n+1 and hence closo structure.

Similarly

find the structures for

i) C

B

H

Answer:

i) C

B

H

- SEP = 8 therefore n+2, nido

Preparation of Carboranes

50 C

1. B

H

+ C

H

nido- 1, 2 - C

B

H

490 C

Properties of Carboranes

i) Closo carboranes are most stable carboranes

ii) The carboranes can show facile electrophilic substitution Br₂

Closo-1, 6-C₂B₇H₉ 8-Br -1, 6-C₂B₇H₈ + HBr AlCl₃

iii) Carboranes react with suitable reagents to give metallic carboranes

C₂B₉H₁₂- _{+ NaH Na}

2C2B9H11 + H2

C₂B₁₀H₁₂ + C₄H₉Li C₂Li₂B₁₀H₁₀ + 2 C₄ H₁₀

BORON NITRIDE

•

Boron nitride, BN is a white solid.

•

It is a giant molecule with a graphite-like structure in which boron and

nitrogen atoms alternate in the rings giving structure with B-N bond

length of 145 pm.

•

The bonds are formed by sp hybrid orbitals on boron and nitrogen

atoms; the remaining electrons are used for π-bond formation.

•

The layers are arranged in a manner so that boron atoms in one layer are

immediately over the nitrogen atoms in an adjacent layer.

It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice.

Amorphous form (a-BN)

The amorphous form of boron nitride (a-BN) is non-crystalline, lacking any long-distance regularity in the arrangement of its atoms. It is analogous to amorphous carbon.

Crystalline form

a) The hexagonal form (h-BN) corresponding to graphite is the most stable and soft among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products.

b) The cubic (sphalerite structure) (c-BN) variety analogous to diamond is called c-BN; it is softer than diamond, but its thermal and chemical stability is superior.

c) The rare wurtzite BN (w-BN) modification is similar to lonsdaleite and may even be harder than the cubic form.

Boron nitride is produced synthetically.

Hexagonal boron nitride is obtained by the reacting boron trioxide (B₂O₃) or boric acid (B(OH)₃) with ammonia (NH₃) or urea (CO(NH₂)₂) in a nitrogen atmosphere:[

B₂O₃ + 2 NH₃ → 2 BN + 3 H₂O (T = 900 °C) B(OH)₃ + NH₃ → BN + 3 H₂O (T = 900 °C)

B₂O₃ + CO(NH₂)₂ → 2 BN + CO₂ + 2 H₂O (T > 1000 °C)

The resulting disordered (amorphous) boron nitride contains 92–95% BN and 5–8%

B₂O₃.

The remaining B₂O₃ can be evaporated in a second step at temperatures > 1500

°C in order to achieve BN concentration >98%. Such annealing also crystallizes BN, the size of the crystallites increasing with the annealing temperature.

Preparation of cubic BN

Synthesis of c-BN uses same methods as that of diamond:

Cubic boron nitride is produced by treating hexagonal boron nitride at high pressure and temperature, much as synthetic diamond is produced from graphite.

Direct conversion of hexagonal boron nitride to the cubic form has been observed at pressures between 5 and 18 GPa and temperatures between 1730 and 3230 °C, that is similar parameters as for direct graphite-diamond conversion

Preparation of wurtzite BN

Combustion of boron powder in nitrogen plasma at 5500 °C yields

ultrafine boron nitride used for lubricants and toners.

Boron nitride reacts with iodine fluoride in trichlorofluoromethane at

−30 °C to produce an extremely sensitive explosive, NI

, in low yield.

Boron nitride reacts with nitrides of alkali metals and lanthanides to

form nitridoborate compounds.For example:

Li

N + BN → Li

BN

• h-BN lubricants can be used even in vacuum, e.g. in space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads

• Because of its excellent thermal and chemical stability, h-BN ceramics are traditionally used as parts of high-temperature equipment. h-BN can be included in ceramics, alloys, resins, plastics, rubbers, and other materials, giving them self-lubricating properties.

• Cubic boron nitride (CBN or c-BN) is widely used as an abrasive. Its usefulness arises from its insolubility in iron, nickel, and related alloys at high temperatures, whereas diamond is soluble in these metals to give carbides

Boron Halides

The halides react with water to form boric acid.

All three lighter boron trihalides (BF

, BCl

, and BBr

) form stable adducts with

common Lewis bases.

The sequence for the Lewis acidity is BF

< BCl

< BBr

,where BBr

is the

strongest Lewis acid. This trend is commonly attributed to the degree of π

-bonding in the planar boron trihalid that would be lost upon pyramidization of

the the BX

molecule.

Boron tribromide (BBr

is a colorless, fuming liquid tshat is an excellent

demethylating or dealkylating agent.

Boron trichloride (BCl

) is a colorless, dangerously reactive gas that is a valuable

reagent in organic synthesis.