size does matter!

One interesting idea in chemistry is about steric hindrance, which refers to the hindrance of chemical action ascribed to the arrangement of atoms in a molecule according to the Webster Dictionary.

What the dictionary did not really highlight but the idea is there is that it is usually the central atom in a molecule that participates in a lot of the reactions that we see around us.

This is especially true for nucleophilic subsititution, which commonly involve attacking an electron deficient carbon atom, and that the atoms surrounding the carbon atom can determine where the nucleophile can attack ther carbon atom.

Another interesting example highlighted here would be that boron halides do not dimerise. Boron is a group 3 element with the atomic number of 5 and it can form “halides” when it combines with group 7 elements to form covalent compounds like BF3 or BCl3.

We know that AlCl3 actually dimerises to form Al2Cl6 in nature so that the aluminium atom in the centre can attain octet electronic configuration. But interestingly enough, BCl3 cannot form dimers, and that is due to steric hindrance. It is just too squeezy or sterically unfavourable to put four large halogen atoms around a boron atom!

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