This reaction may be either a homopolymerization of a single monomer A-B with two different end groups that condense, or a copolymerization of two co-monomers A-A and B-B. Small molecules are usually liberated in these condensation steps, unlike polyaddition reactions. Condensation polymers often require heat, form slower than do addition polymers, and are lower in molecular weight.
This type of reaction is used as a basis for making many important polymers, such as nylon, polyester, and various epoxies. It is also the basis for the laboratory formation of silicates and polyphosphates. Many biological transformations, such as polypeptide synthesis, polyketide synthesis, terpene syntheses, phosphorylation, and glycosylations are condensations.
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:. Skip to main content. Search for:. Condensation Reactions. Learning Objective Recognize the chemical principles of condensation reactions as they relate to polymerization. Key Points During condensation reaction, two molecules combine to form a single molecule with the loss of a small molecule; in dehydration reaction, this lost molecule is water.
Intermolecular condensation occurs between two separate molecules, while intramolecular condensation is the union between atoms or groups of the same molecule, often leading to ring formation.
Different types of alcohols may dehydrate through a slightly different mechanism pathway. This ion acts as a very good leaving group which leaves to form a carbocation. The deprotonated acid the base then reacts with the hydrogen adjacent to the carbocation and form a double bond. Primary alcohols undergo bimolecular elimination E2 mechanism while secondary and tertiary alcohols undergo unimolecular elimination E1 mechanism.
The relative reactivity of alcohols in dehydration reactions is ranked as follows:. Primary alcohols dehydrate through the E2 mechanism. The hydroxyl oxygen donates two electrons to a proton from sulfuric acid H 2 SO 4 , forming an alkyloxonium ion.
Then the conjugate base, HSO 4 — , reacts with one of the adjacent beta hydrogen atoms while the alkyloxonium ion leaves in a concerted process, forming a double bond. Secondary and tertiary alcohols dehydrate through the E1 mechanism.
Similarly to the reaction above, secondary and tertiary —OH protonate to form alkyloxonium ions. However, in this case the ion leaves first and forms a carbocation as the reaction intermediate.
The water molecule which is a stronger base than the HSO 4 - ion then abstracts a proton from an adjacent carbon to form a double bond.
Notice in the mechanism below that the alkene formed depends on which proton is abstracted: the red arrows show formation of the more substituted 2-butene, while the blue arrows show formation of the less substituted 1-butene. Recall that according to Zaitsev's Rule, the more substituted alkenes are formed preferentially because they are more stable than less substituted alkenes.
Additinally, trans alkenes are more stable than cis alkenes and are also the major product formed. For the example below, the trans diastereomer of the 2-butene product is most abundant.
The dehydration mechanism for a tertiary alcohol is analogous to that shown above for a secondary alcohol. Examples of these and related reactions are given in the following figure. The predominance of the non-Zaitsev product less substituted double bond is presumed due to steric hindrance of the methylene group hydrogen atoms, which interferes with the approach of base at that site.
The first uses the single step POCl3 method, which works well in this case because SN2 substitution is retarded by steric hindrance. The second method is another example in which an intermediate sulfonate ester confers halogen-like reactivity on an alcohol.
In every case the anionic leaving group is the conjugate base of a strong acid. In the dehydration of this diol the resulting product is a ketone. Draw the mechanism of its formation.
Hint a rearrangement occurs. Draw an arrow pushing mechanism for the acid catalyzed dehydration of the following alcohol, make sure to draw both potential mechanisms.
Assume no rearrangement for the first two product mechanisms.
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