Lone pair bond formed from

Boric acid, B(OH) , is a white solid that melts at 171°C. It is toxic to bacteria and many insects as well as humans and has long been used as a mild antiseptic and pesticide. Because the boron atom in B(OH)3 has an incomplete octet, it can act as a Lewis acid and form a bond by accepting a lone pair of electrons from an H20 molecule acting as a Lewis base ... 

Answer We notice that this is a lone pair next to a pi bond. Therefore, we push two arrows one from the lone pair to form a pi bond, and one from the pi bond to form a lone pair. 

Notice that there is one long flow of electron density, illustrated with three arrows. We begin at the tail of the arrow on the base, because that is where the flow starts. This arrow is going from a lone pair to form a bond. The second arrow goes from a bond to form a bond, and the third arrow goes from a bond to form a lone pair on X. This type of reaction is called an elimination reaction, because we are eliminating H+ and X to form a double bond ... 

In the second step of the mechanism, we also have two arrows from a lone pair to form a bond, and then from a bond to form a lone pair ... 

Answer The first arrow is going from a lone pair on the sulfur atom to form a bond between sulfur and carbon. So, this arrow is of the type lone pair bond. 

A hydrogen bond is formed between a hydrogen atom and a lone pair electrons from an atom in a neighboring molecule. For example, the hydrogen atom of a water molecule forms a hydrogen bond with the lone pair of electrons from an oxygen atom in another water molecule. 

Lone pair electrons may be used in bonding. Since they already have a complement of two electrons, bonds will need to be made to an atom that is electron deficient, e.g. a proton. Thus, the ammonium cation and the hydronium cation also share tetrahedral geometry, and each possesses a a bond formed from lone pair electrons. 

Alkyl halides (RX) are good substrates for substitution reactions. The nucleophile (Nu ) displaces the leaving group (X ) from the carbon atom by using its electron parr or lone pair to form a new a bond to the carbon atom. Two different mechanisms for nucleophilic substitution are SnI and 8 2 mechanisms. In fact, the preference between S l and 8 2 mechanisms depends on the structure of the alkyl halide, the reactivity and structure of the nucleophile, the concentration of the nucleophile and the solvent in which reaction is carried out. 

The acylation is thought to occur as shown in Scheme 3. The ir-allyl intermediate (5) was isolated in 86% yield and characterized by X-ray analysis.13 The NMR spectrum and crystal structure of (5) indicate that only the anti-al y structure is formed no other positional isomers were detected. The iron atom in (5) obtains a closed-shell electron configuration by intramolecular o-donation of a lone pair of electrons from the acyl oxygen atom, which is confirmed by a lowering the acyl CO bond order (vco 1637 cm-1).12 There is no direct evidence for formation of (4) its intermediacy has been suggested primarily based on the endo stereochemistry of the acylation reaction (see below). 

Step 5. If no unassigned electrons remain after step 3 but the central atom does not yet have an octet, use one or more lone pairs of electrons from a neighboring atom to form a multiple bond (either double or triple). Oxygen, carbon, nitrogen, and sulfur often form multiple bonds. Worked Example 7.7 shows how to deal with such a case. 

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