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The structure of 3-methyl-2-hexenoic acid.

Boron trihalides: a special case

The three lighter boron trihalides, BX 3 (X = F, Cl, Br) form stable adducts with common Lewis bases. Their relative Lewis acidities can be evaluated in terms of the relative exothermicity of the adduct-forming reaction:

This trend is opposite to that expected based upon the electronegativity of the halogens. The best explanation of this trend takes into account the extent of π-donation that occurs between the filled lone pair orbital on the halogens and the empty p-orbital on the planar boron ( [link] ). As such, the greater the π-bonding the more stable the planar BX 3 configuration as opposed to the pyramidalization of the BX 3 moiety upon formation of a Lewis acid-base complex, [link] .

Schematic representation of π-donation from filled halogen p-orbitals into empty p-orbital in the halides BX 3 .

The criteria for evaluating the relative strength of π-bonding are not clear, however, one suggestion is that the F atom is small compared to the Cl atom, and the lone pair electron in p z of F is readily and easily donated and overlapped to empty p z orbital of boron ( [link] a). In contrast, the overlap for the large (diffuse) p-orbitals on the chlorine is poorer ( [link] b). As a result, the π-donation of F is greater than that of Cl. Interestingly, as may be seen from [link] , any difference in B-X bond length does not follow the expected trend associated with shortening of the B-X bond with stronger π-bonding. In fact the B-Br distance is the most shortened as compared to that expected from the covalent radii ( [link] ).

The B-X bond distances in the boron trihalides, BX 3 , as compared to the sum of the covalent radii. a Covalent radius of B = 0.84(3) Å.
Compound B-X (Å) X covalent radius (Å) Sum of covalent radii (Å) a Δ (Å)
BF 3 1.313 0.57(3) 1.41 0.097
BCl 3 1.75 1.02(4) 1.86 0.11
BBr 3 1.898 1.20(3) 2.04 0.142
BI 3 2.125 1.39(3) 2.23 0.105

At the simplest level the requirements for bonding to occur based upon the molecular or atomic orbital are:

  • Directional relationship of the orbitals.
  • Relative symmetry of the orbitals.
  • Relative energy of the orbitals.
  • Extent of orbital overlap

In the case of the boron trihalides, the direction (parallel) and symmetry (p-orbitals) are the same, and the only significant difference will be the relative energy of the donor orbitals (i.e., the lone pair on the halogen) and the extent of the overlap. The latter will be dependant on the B-X bond length (the shorter the bond the greater the potential overlap) and the diffusion of the orbitals (the less diffuse the orbitals the better the overlap). Both of these factors will benefit B-F over B-Cl and B-Br. Thus, the extent of potential overlap would follow the order: [link] . Despite these considerations, it is still unclear of the exact details of the rationalization of the low Lewis basicity of BF 3 as compared to BCl 3 and BBr 3 .

Anionic halides

The trihalides all form Lewis acid-base complexes with halide anions, [link] , and as such salts of BF 4 - , AlCl 4 - , GaCl 4 - , and InCl 4 - are common.

In the case of gallium the Ga 2 Cl 7 - anion ( [link] ) is formed from the equilibrium:

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
waht is hydrating power of lithium carbonates
Mahar Reply

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Source:  OpenStax, Chemistry of the main group elements. OpenStax CNX. Aug 20, 2010 Download for free at http://cnx.org/content/col11124/1.25
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