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An alkoxide is an organic functional group that is formed by the deprotonation of the hydroxyl group of an alcohol, leaving the oxygen with a negative charge. From the point of view of the Brønsted and Lowry acid/base theory, an alkoxide is nothing more than the conjugate base of an alcohol.
Structurally, alkoxides are alkyl groups attached to a negatively charged oxygen (hence the ending –oxide). Negative oxygen easily binds to metal cations through a bond that is more covalent than ionic, thus forming neutral Organometallic compounds, however, it behaves in organic solutions as a good source of alkoxide ions in a similar way to that of an organic salt.
Alkoxides are of immense importance in organic chemistry since they are used in a myriad of synthetic routes that require strong bases or good nucleophiles, either to carry out elimination reactions (such as the dehydrohalogenation reactions of alkyl halides) or nucleophilic substitution.
Formation of alkoxides
Alcohols are very weak acids. In fact, they are even weaker acids than water, so they cannot be made by reacting them with a strong base in aqueous solution, since any base that dissolves in water will immediately react with it before alcohol.
There are three common ways to synthesize alkoxides:
The reaction of an alcohol with an alkali metal
The most common way to prepare alkoxides is by reacting the alcohol with an alkali metal such as lithium, sodium, or potassium under anhydrous conditions (in the complete absence of water). The reaction generates an alkoxide of the metal and gaseous hydrogen:
Neutralization of an alcohol with a strong base
The second way to obtain alkoxides, which is much cheaper than the first, is by reacting an alcohol with a strong base under anhydrous conditions by means of an acid-base neutralization reaction.
salt metathesis
Double replacement or metathesis reactions can be used to replace sodium, lithium, or potassium in an easy-to-prepare alkoxide with another metal of interest.
Properties of alkoxides
Some of the properties that make alkoxides very frequently used in organic synthesis are:
- They are very strong bases. This makes them useful for carrying out elimination reactions on alkyl halides.
- If R is small, they are good nucleophiles. This is very useful for reactions such as transesterification.
- They are soluble in many organic solvents, unlike strong mineral bases which are only soluble in water and highly polar organic solvents.
- They are solid at room temperature.
Examples of common alkoxides in organic synthesis
As mentioned above, alkoxides are widely used in synthetic organic chemistry. Here is a list of some of the most commonly used alkoxides for this purpose.
Sodium tertbutoxide (C 4 H 9 ONa)
Derived from tert-butanol, this bulky alkoxide is the perfect choice when organic chemists need a good base but not a good nucleophile. It is frequently used as a catalyst base for dehydrohalogenation reactions of primary alkyl halides.
Sodium methoxide or sodium methylate (CH 3 ONa)
This is the smallest and simplest alkoxide of all. Unlike tert-butoxide, which is very bulky, preventing it from acting as a good nucleophile, sodium methoxide is both a strong base and a good nucleophile.
Sodium ethoxide (C 2 H 5 ONa)
It is also in turn a good base and a good nucleophile. It is used in the malon synthesis as well as in the Claisen condensation reaction.
