Writing Organic Reactions

This is a good time to mention that organic reaction equations are sometimes written in different ways to emphasize different points. For example, the reaction of ethylene with HBr might be written in the format A + B — C to emphasize that both reactants are equally important for the purposes of the discussion. The solvent and notes about other reaction conditions such as temperature are vsrritten either above or below the reaction arrow. 

Alternatively, we might write the same reaction in the format 

Both reaction formats are frequently used in chemistry, and you sometimes have to look carefully at the overall transformation to see the roles of the substances above and below the reaction arrow. 

Alternatively, we might write the same reaction in a format to emphasize that 2-methylpropene is the reactant whose chemistry is of greater interest. The second reactant, HCl, is placed above the reaction arrow together with notes about solvent and reaction conditions  

After looking at the results of many such reactions, the Russian chemist Vladimir Markovnikov proposed in 1869 what has become known as Markovnikov s rule  

In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents. 

When both double-bond carbon atoms have the same degree of substitution, a mixture of addition products results  

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Every once in a while you may run into an organic chemistry problem in Section II, on writing equations. An example is Write an equation that describes burning methanol in air. You would need to know the chemical formula and structural formula of methanol in order to do this problem. Writing organic reactions is covered in more detail in the chapter entitled Writing and Predicting Chemical Reactions. ... 

The fourteenth century English philosopher William of Occam introduced the principle known as Occam s razor. A paraphrase of this principle which can be applied to writing organic reaction mechanisms is expressed in Hint 2.17. 

Electrophilic Addition of HX to Alkenes 206 Writing Organic Reactions 208... 

This is an extension of the skeletal notation. It is very good at indicating the structure of ring compounds and the cyclic transition states of aliphatic compounds, and so is particularly useful when writing organic reaction mechanisms that involve such species. 

For most students, common practices in writing organic reactions will be different than used in general chemistry. 

One final comment on mechanisms in gencrtil There exists a common sort of shorthand way of writing organic reaction mechanisms. First, each step is written separately. Second, bonding changes in each step are indicated by arrows that represent the movement of pairs of electrons. For the Sn2 mechanism, a one-sicp process, we have ... 

Reversing the direction of one or more arrows during a chemical step is the most common mistake made by students when writing organic reaction mechanisms. Backward arrow pushing usually derives from a student thinking about the movement of atoms, not the movement of electrons. Hence, to avoid this mistake it is important to remember that arrows depict how electrons move, not where atoms move, within or between chemical structures. Further, one can avoid this mistake by remembering that every arrow must start at an electron source (a bond or lone pair) and terminate at an electron sink (an atom that can accept a new bond or lone pair). 

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