- Structure of a secondary alcohol
- Properties
- Boiling points
- Acidity
- Reactions
- Dehydration
- Reaction with active metals
- Esterification
- Oxidation
- Nomenclature
- Applications
- Examples
- 2-Octanol
- Estradiol or 17β-Estradiol
- 20-Hydroxy-leukotriene
- 2-Heptanol
- References
A secondary alcohol has the carrier carbon for the hydroxyl group (OH), attached to two carbons. Meanwhile, in primary alcohol the carbon bearing the hydroxyl group is attached to one carbon atom, and in tertiary alcohol attached to three carbon atoms.
Alcohols are slightly weaker acids than water, with the following pKa: water (15.7); methyl (15.2), ethyl (16), isopropyl (secondary alcohol, 17), and tert-butyl (18) alcohols. As can be seen, isopropyl alcohol is less acidic than methyl and ethyl alcohols.
Structural formula of a secondary alcohol. Source: Jü, from Wikimedia Commons
The upper image shows the structural formula for a secondary alcohol. The carbon in red is the carrier of the OH, and is linked to two alkyl (or aryl) R groups and to a single hydrogen atom.
All alcohols have the generic formula ROH; but if the carrier carbon is observed in detail, then the primary (RCH 2 OH), secondary (R 2 CHOH, here raised), and tertiary (R 3 COH) alcohols are obtained. This fact makes a difference in its physical properties and reactivity.
Structure of a secondary alcohol
The structures of the alcohols depend on the nature of the R groups. However, for secondary alcohols some exemplification can be made, considering that there can only be linear structures with or without branches, or cyclic structures. For example, you have the following image:
A cyclic secondary alcohol and a branched-chain secondary alcohol. Source: Gabriel Bolívar
Note that for both structures there is something in common: OH is linked to a "V". Each end of the V represents an equal R group (upper part of the image, cyclic structure) or different (lower part, branched chain).
In this way any secondary alcohol is very easily identified, even if its nomenclature is unknown at all.
Properties
Boiling points
The properties of secondary alcohols physically do not differ too much from other alcohols. They are usually transparent liquids, and to be a solid at room temperature it needs to form several hydrogen bonds and have a high molecular mass.
However, the same structural formula R 2 CHOH suggests certain generally unique properties for these alcohols. For example, the OH group is less exposed and available for hydrogen bonding interactions, R 2 CH-OH-OHCHR 2.
This is because the R groups, adjacent to the OH-bearing carbon, can get in the way and hinder the formation of hydrogen bonds. As a result, secondary alcohols have lower boiling points than primary ones (RCH 2 OH).
Acidity
According to the Brönsted-Lowry definition, an acid is one that donates protons or hydrogen ions, H +. When this happens with a secondary alcohol, you have:
R 2 CHOH + B - => R 2 CHO - + HB
The conjugate base R 2 CHO -, an alkoxide anion, must stabilize its negative charge. For secondary alcohol, the stabilization is less since the two R groups have electronic density, which repels to a certain degree the negative charge on the oxygen atom.
Meanwhile, for the alkoxide anion of a primary alcohol, RCH 2 O -, there is less electronic repulsion as there is only one R group and not two. Furthermore, hydrogen atoms do not exert significant repulsion, and on the contrary, contribute to stabilize the negative charge.
Therefore, secondary alcohols are less acidic than primary alcohols. If this is so, then they are more basic, and for exactly the same reasons:
R 2 CHOH + H 2 B + => R 2 CHOH 2 + + HB
Now, the R groups stabilize the positive charge on oxygen by giving up part of its electron density.
Reactions
Hydrogen halide and phosphorus trihalogenide
A secondary alcohol can react with a hydrogen halide. The chemical equation of the reaction between isopropyl alcohol and hydrobromic acid, in a sulfuric acid medium, and producing isopropyl bromide is shown:
CH 3 CHOHCH 3 + HBr => CH 3 CHBrCH 3 + H 2 O
And it can also react with a phosphorous trihalogen, PX 3 (X = Br, I):
CH 3 -CHOH-CH 2 -CH 2 -CH 3 + PBr 3 => CH3-CHBr-CH 2 -CH 2 -CH 3 + H 3 PO 3
The above chemical equation corresponds to the reaction between Sec-pentanol and phosphorous tribromide, resulting in sec-pentyl bromide.
Note that in both reactions a secondary alkyl halide (R 2 CHX) is produced.
Dehydration
In this reaction, an H and an OH from neighboring carbons are lost, forming a double bond between these two carbon atoms. Therefore, there is the formation of an alkene. The reaction requires an acid catalyst and the supply of heat.
Alcohol => Alkene + H 2 O
For example, we have the following reaction:
Cyclohexanol => Cyclohexene + H 2 O
Reaction with active metals
Secondary alcohols can react with metals:
CH 3 -CHOH-CH 3 + K => CH 3 CHO - K + CH 3 + ½ H +
Here isopropyl alcohol reacts with potassium to form the isoproxide salt of potassium and hydrogen ions.
Esterification
The secondary alcohol reacts with a carboxylic acid to form an ester. For example, the chemical equation for the reaction of sec-butyl alcohol with acetic acid to produce sec-butyl acetate is shown:
CH 3 CHOHCH 2 CH 3 + CH 3 COOH <=> CH 3 COOCHCH 3 CH 2 CH 3
Oxidation
Primary alcohols are oxidized to aldehydes, and these in turn are oxidized to carboxylic acids. But, secondary alcohols are oxidized to acetone. The reactions are usually catalyzed by potassium dichromate (K 2 CrO 7) and chromic acid (H 2 CrO 4).
The overall reaction is:
R 2 CHOH => R 2 C = O
Nomenclature
Secondary alcohols are named by indicating the position of the OH group in the main (longest) chain. This number precedes the name, or it can come after the name of the respective alkane for that chain.
For example, CH 3 CH 2 CH 2 CH 2 CHOHCH 3 is 2-hexanol or hexan-2-ol.
If the structure is cyclical, there is no need to place a numerator; unless there are other substituents. This is why the cyclic alcohol in the second image is called cyclohexanol (the ring is hexagonal).
And for the other alcohol in the same image (the branched one), its name is: 6-ethyl-heptan-2-ol.
Applications
-Sec-butanol is used as solvent and chemical intermediate. It is present in hydraulic fluids for brakes, industrial cleaners, polishes, paint strippers, mineral flotation agents, and fruit essences and perfumes.
-Isopropanol alcohol is used as an industrial solvent and as an anticoagulant. It is used in oils and quick-drying inks, as an antiseptic and substitute for ethanol in cosmetics (for example, skin lotions, hair toners, and rubbing alcohol).
-Isopropanol is an ingredient in liquid soaps, glass cleaners, synthetic flavorings in non-alcoholic beverages and foods. Also, it is a chemical intermediate.
-Cyclohexanol is used as a solvent, in fabric finishing, in leather processing and as an emulsifier in soaps, and synthetic detergents.
-Methylcyclohexanol is an ingredient in soap-based stain removers and special fabric detergents.
Examples
2-Octanol
2-octanol molecule. Source: Jü, from Wikimedia Commons
It is a fatty alcohol. It is a colorless liquid, slightly soluble in water, but soluble in most non-polar solvents. It is used, among other uses, in the production of flavors and fragrances, paints and coatings, inks, adhesives, home care and lubricants.
Estradiol or 17β-Estradiol
Estradiol molecule. Source: NEUROtiker, from Wikimedia Commons
It is a steroid sex hormone. It has two hydroxyl groups in its structure. It is the predominant estrogen during the reproductive years.
20-Hydroxy-leukotriene
It is a metabolite that probably originates from the oxidation of the lipid of leukotriene. It is classified as a cystinyl leukotriene. These compounds are mediators of the inflammatory process that contributes to the pathophysiological characteristics of allergic rhinitis.
2-Heptanol
It is an alcohol found in fruits. Also, it is found in ginger oil and strawberries. It is transparent, colorless and insoluble in water. It is used as a solvent for various resins and is involved in the flotation phase in mineral processing.
References
- James. (September 17, 2014). Alcohols (1) - Nomenclature and Properties. Recovered from: masterorganicchemistry.com
- Encyclopedia of Health and Safety at Work. (sf). Alcohols.. Recovered from: insht.es
- Clark J. (July 16, 2015). Structure and Classification of Alcohols. Chemistry Libretexts. Recovered from: chem.libretexts.org
- PubChem. (2019). 20-Hydroxy-leukotriene E4. Recovered from: pubchem.ncbi.nlm.nih.gov
- Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5 ta Edition. Editorial Addison-Wesley Interamericana.
- Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
- Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10 th edition.). Wiley Plus.
- Wikipedia. (2018). 2-Octanol. Recovered from: en.wikipedia.org