Borane is an

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

. Because it tends to dimerize or form

adduct In chemistry, an adduct (; alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is ...

s, borane is very rarely observed. It normally dimerizes to diborane in the absence of other chemicals. It can be observed directly as a continuously produced, transitory, product in a flow system or from the reaction of laser ablated atomic boron with hydrogen.

Structure and properties

BH₃ is a trigonal planar molecule with D_3h symmetry. The experimentally determined B–H bond length is 119 pm. In the absence of other bases, it dimerizes to form diborane. Thus, it is an intermediate in the preparation of diborane according to the reaction: :BX₃ +BH₄⁻ → HBX₃⁻ + (BH₃) (X=F, Cl, Br, I) :2 BH₃ → B₂H₆ The standard enthalpy of dimerization of BH₃ is estimated to be −170 kJ mol⁻¹. The boron atom in BH₃ has 6 valence electrons. Consequently, it is a strong

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...

and reacts with any Lewis base ('L' in equation below) to form an adduct: :BH₃ + L → L—BH₃ in which the base donates its lone pair, forming a dative

covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

. Such compounds are thermodynamically stable, but may be easily oxidised in air. Solutions containing borane dimethylsulfide and borane–tetrahydrofuran are commercially available; in tetrahydrofuran a stabilising agent is added to prevent the THF from oxidising the borane.Hydrocarbon Chemistry, George A. Olah, Arpad Molner, 2d edition, 2003, Wiley-Blackwell A stability sequence for several common adducts of borane, estimated from spectroscopic and thermochemical data, is as follows: : PF₃ < CO< Et₂O< Me₂O< C₄H₈O < C₄H₈S < Et₂S< Me₂S< Py < Me₃N< H⁻ BH₃ has some soft acid characteristics as sulfur donors form more stable complexes than do oxygen donors. Aqueous solutions of BH₃ are extremely unstable. : + 3 → +

Reactions

Molecular species BH₃ is a very strong

. It can be isolated in the form of various adducts, such as borane carbonyl, BH₃(CO). Molecular BH₃ is believed to be a reaction intermediate in the

pyrolysis Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology The word ''pyrolysis'' is coined from the Gree ...

of diborane to produce higher boranes: :B₂H₆ ⇌ 2BH₃ :BH₃ +B₂H₆ → B₃H₇ +H₂ (rate determining step) :BH₃ + B₃H₇ ⇌ B₄H₁₀ :B₂H₆ + B₃H₇ → BH₃ + B₄H₁₀ :::::⇌ B₅H₁₁ + H₂ Further steps give rise to successively higher boranes, with B₁₀H₁₄ as the most stable end product contaminated with polymeric materials, and a little B₂₀H₂₆. Borane ammoniate, which is produced by a displacement reaction of other borane adducts, eliminates elemental hydrogen on heating to give

borazine Borazine, also known as borazole, inorganic benzene, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benz ...

(HBNH)₃. Borane adducts are widely used in

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

for hydroboration, where BH₃ adds across the C=C bond in

alkenes In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

to give trialkylboranes: :(THF)BH₃ + 3 CH₂=CHR → B(CH₂CH₂R)₃ + THF This reaction is regioselective. Other borane derivatives can be used to give even higher regioselectivity. The product trialkylboranes can be converted to useful organic derivatives. With bulky alkenes one can prepare species such as BR₂sub>2, which are also useful reagents in more specialised applications. Borane dimethylsulfide which is more stable than borane–tetrahydrofuran may also be used. Hydroboration can be coupled with oxidation to give the hydroboration-oxidation reaction. In this reaction, the boryl group in the generated organoborane is substituted with a

hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

As a Lewis acid

Phosphine-boranes, with the formula R_3−nH_nPBH₃, are adducts of organophosphines and borane. Borane adducts with amines are more widely used. Borane makes a strong adduct with

triethylamine Triethylamine is the chemical compound with the formula N(CH2CH3)3, commonly abbreviated Et3N. Like triethanolamine and the tetraethylammonium ion, it is often abbreviated TEA. It is a colourless volatile liquid with a strong fishy odor remini ...

; using this adduct requires harsher conditions in hydroboration. This can be advantageous for cases such as hydroborating trienes to avoid polymerization. More sterically hindered tertiary and silyl amines can deliver borane to alkenes at room temperature. Examples of amine borane complexes

Borane(5) is the

dihydrogen complex Dihydrogen complexes are coordination complexes containing intact H2 as a ligand. They are a subset of sigma complexes. The prototypical complex is W(CO)3(Tricyclohexylphosphine, PCy3)2(H2). This class of chemical compound, compounds represent in ...

of borane. Its molecular formula is BH₅ or possibly BH₃(η²-H₂). It is only stable at very low temperatures and its existence is confirmed in very low temperature. Borane(5) and

methanium In chemistry, methanium is a complex positive ion with formula (metastable transitional form, a carbon atom covalently bonded to five hydrogen atoms) or (fluxional form, namely a molecule with one carbon atom covalently bonded to three hydro ...

(CH₅⁺) are

isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...

.A Life of Magic Chemistry: Autobiographical Reflections Including Post-Nobel Prize Years and the Methanol Economy, 159p Its

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid gives a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as it loses a hydrogen ion in the reve ...

is the

borohydride Borohydride refers to the anion , which is also called tetrahydroborate or more commonly tetrahydrobiopterin, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for ex ...

anion.

References

{{Hydrides by group

Structure and properties

Reactions

As a Lewis acid

See also

References