III. ORGANIC CHEMISTRY: Reactions

Read Section 1.3 (Nelson Chemistry 12)

Additional Note: * Note the typos on p. 25: the superscript 2 and subscript z should all be subscript 2.

Types of Reactions

Complete Combustion Reactions

One of the most common types of reactions with organic compounds is the combustion reaction. Any organic compound can undergo a combustion reaction with oxygen gas to form a variety of products. However, the one that we will focus on is the complete combustion reaction, which is the only one for which we can predict the outcome.

The combustion of organic compounds is an example of a decomposition reaction, because more complex compounds are turned into simpler compounds.

The combustion reaction in organic chemistry is also an example of a reaction that bridges the gap between the organic chemistry and inorganic chemistry.

When writing a complete combustion equation for organic chemistry, the structural formula is used for the organic compound (the fuel) only; structural formulas are not required for inorganic compounds. As well, molecular formulas are the better choice for all compounds in the equation for balancing purposes.

Fractions are left in complete combustions equations, because in most cases they are written as thermochemical equations.

Many organic compounds can be produced, or synthesized, from a few basic types of reactions:

Substitution Reactions

halogens or groups of elements, such as hydroxyl, amino, or nitro groups, replace a hydrogen atom on a hydrocarbon chain

hydrocarbons are relatively inert (very stable) at SATP; therefore, special conditions are most often required for a substitution to occur (to “activate” a region on the parent chain)

substitution reactions occur one step at a time; therefore, two substitutions cannot (and do not) take place at the same time (ex. bromine is diatomic, meaning two atoms of bromine are available for a substitution; however, each bromine gets added to a separate hydrocarbon, yielding a HBr molecule in each case, and not two bromines onto the same hydrocarbon, yielding a H2 molecule)

note the conditions and experimental evidence of substitution reactions, as described on p. 24-25, very carefully.

note the functional groups in Table 2 (p. 25). These are organized according to decreasing preference for bonding with C in a substitution reaction. At the bottom, but not listed, would be hydrogen. You can use this “hierarchy” to predict whether or not a substitution will take place. For example, Br can substitute OH on a hydrocarbon, but Br cannot substitute the more reactive Cl.

The substitution of hydrogen on a benzene ring requires more energy than the same reaction with and aliphatic, saturated ring, but less energy than the same reaction on an aliphatic unsaturated ring (see p. 28).

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Addition Reactions

require non-aromatic sites of pi bonding

unsaturates undergo addition reactions more easily than substitution reactions because pi bonds are more easily disrupted than sigma bonds (especially in hydrocarbon chains)

all addition reactions proceed to complete saturation (i.e. an alkyne will not react partially to form an alkene – it will react all the way to form an alkane)

always take note of the catalysts and conditions required for a reaction to occur; for example, Br2 and Cl2 are easily disrupted in the presence of double and triple bonds – addition reactions with these occur readily; H2 and H2O are much more stable and therefore reactions involving these still need a boost of energy or the help of a catalyst.

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although benzene (phenyl group) contains double bonds, the delocalization of the pi electrons does not allow for the same reactivity you would see in non-aromatic unsaturates. That is, delocalization adds a special reaction-resistant stability to the benzene ring, rendering it much less reactive than an alkene or alkane.

You can then conclude that benzene is both highly reactive (through substitution) and very unreactive (through addition).

Complete the following: Answer: #1 – 3 (p. 27); #4 – 7 (p. 30); #1 – 4 (p. 31)

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Condensation Reactions

involve the synthesis of a larger product molecule from smaller reactant molecules

also known as “dehydration” reactions, because a water molecule is removed to link the smaller reactant molecules

a “linkage” structure is evident where the synthesis took place

A. Formation of an Ether from Alcohols (Ether Synthesis)

B. The Formation of an Ester from a Carboxylic Acid and an Alcohol (Ester Synthesis)(aka Esterification)

An example is the esterification of ethanol with butanoic acid. Draw/write it below:

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