Benzoic Acid - Structure and Properties
Benzoic acid is used in a number of daily activities and is a very commonly seen product in many industrial as well as household applications. The structure of benzoic acid is a carboxyl group attached to a ring of benzene. It is usually denoted using the formula C6H5COOH. It contains an aromatic ring and therefore is also referred to as aromatic benzoic acid. As can probably be understood from this, this is also the reason why this compound has a certain pleasant smell.
What is the Structure Of Benzoic Acid?
The molecular formula for benzoic acid is C6H5COOH and its empirical formula is C7H6O2. Iot is made up of six atoms of hydrogen, 7 atoms of carbon and 2 atoms of oxygen. There are six carbon rings that have alternate single and double covalent bonds and the -COOH group with it.
What are the Physical Properties of Benzoic Acid?
The following properties can be observed for benzoic acid:
It is a colourless solid crystalline compound in the solid state
Benzoic acid has a molar mass of 122.12 g/mol.
The boiling point of benzoic acid is 523 Kelvin, while its melting point is 395 Kelvin.
Benzoic acid is not soluble in water but it can dissolve in organic solvents.
Chemical Properties
These properties are based on an assumption that each molecule is made up of an aromatic ring and an acidic carboxyl group linked to that ring structure. This benzene ring can be subjected to some of the reactions like nitration, halogenation and sulfonation. Like this, the carboxyl group can undergo some of the reactions such as the formation of esters, acid halides and salts. In particular, these reactions imply modifications of the benzene ring as well as the carboxyl group.
Formation of Salts
The carboxyl group present in benzoic acid is acidic in nature, and thus, reacts with base compounds such as sodium hydroxide to produce a salt known as sodium benzoate (as per the neutralisation reaction). When sodium benzoate is treated with an acid, such as hydrochloric acid, then benzoic acid is formed back.
Formation of Esters
Benzoic acid can also undergo esterification and produce pleasant smelling salts if it reacts with any alcohol, with sulphuric acid acting as a dehydrating agent.
Formation of Acid Halides
When benzoic acid reacts with thionyl chloride (SOCl2) or Phosphorus pentachloride (PCl5), it forms benzoyl chloride that is an example of acyl or acid halides. Benzoyl chloride is highly reactive and so it reacts with amine such as methyl amine (CH3NH2) or with ammonia (NH3) to form benzamide (C6H5CONH2).
Sulfonation
When benzoic acid reacts with the fuming sulfuric acid (H2SO4) it leads to the sulfonation of benzene ring, in which the functional group SO3H replaces H- atom in the meta position to the carboxylic acid group on the benzene ring, and so the product is termed as meta-sulfobenzoic acid.
Nitration
With nitrating agent, sulfuric acid acting as a catalyst, the nitro (NO2) group occupies the meta position (over ortho or para position) relative to the position of the carboxyl group in the benzene ring.
Halogenation
During the halogenation of benzoic acid, chlorine (Cl2) reacts with benzoic acid in presence of a catalyst ferric chloride (FeCl3) to form meta chlorobenzoic acid (Cl-C6H4-COOH).
Decarboxylation of Benzoic Acid
Decarboxylation is the removal of carbon dioxide as sodium carbonate (Na2CO3) by treating the sodium salt of benzoic acid (sodium benzoate) using soda lime (mixture of NaOH & CaO) to form benzene.
Reduction of Benzoic Acid
When benzoic acid is treated with lithium aluminium hydride (LiAlH4) followed by hydrolysis in presence of acid, we will get benzyl alcohol as the product and it lacks selectivity. We cannot use sodium borohydride (NaBH4) for the reduction of carboxylic acids, since it is a weak reducing agent.
Dehydration Reaction
When 2 molecules of carboxylic acid or benzoyl chloride is treated with phosphorus pentoxide P2O5 or acetic anhydride , which acts as a dehydrating agent, removes a water molecule and forms anhydride. If benzoic acid is treated with acetic anhydride (Ac2O), we will get benzoic acid anhydride as the product.
Reduction of Benzoic Anhydride
When the benzoic anhydride is treated with sodium borohydride followed by acid hydrolysis, it is converted into benzoic acid and benzyl alcohol.
Controlled Oxidation of Benzyl Alcohol
When benzoic acid is treated with lithium aluminium hydride it is converted into benzyl alcohol and on controlled oxidation it gets converted into benzaldehyde.
Uses of Benzoic Acid
Some esters of benzoic acid acts as plasticizers
Benzoic acid and its salt sodium benzoate are used as food preservatives for many of the food items like soft drinks, barbecue sauces, pickles and salad dressings which prevents the growth of bacteria and fungi.
Benzoic acid is used in cosmetics as it can specifically help to treat skin irritation, sun burns, insect bites, fungal infections and so on.It is used in the preparation of fragrances and also acts as a pH adjuster.
It is mostly utilized in acidic foods as it has more antifungal and antibacterial properties even at very low pH. It is used as a preservative in acidic medium, but in alkaline medium it loses its activity of preservation since in alkaline medium it splits up into ions.
Benzoic acid is used as a preservative since its addition doesn't alter the odor or taste of the food items.
Its preserving quality delays the multiplication of microorganisms. Sodium salt of benzoic acid, sodium benzoate is used to lower the glycine levels in the blood, since an amide bond forms between the benzoate and glycine, and so there is hippuric acid formation which is excreted through urine.
FAQs on Benzoic Acid
1. Is benzoic acid completely insoluble in water?
Benzoic acid is not soluble in water. Rather, it is soluble in certain organic solvents,, such as alcohols, ethers and benzene. However, water is a universal solvent. This means that water has the ability to dissolve everything in it, irrespective of how long it may take. For benzoic acid, in particular, the insolubility in water does not always remain the same. When temperature is increased, the solubility of benzoic acid in water rises by a certain margin. Therefore, it is not completely insoluble in water.
2. Is benzoic acid harmful to humans?
Benzoic acid is after all an acid, and it is quite a strong acid too. This is why people handling this should be careful to avoid any kinds of chemical burns. Exposure to benzoic acid can lead to irritation in the nose and the airway passage, cause damage to the eye, make the skin red and itchy. In case this is inhaled, then it can lead to coughing or wheezing. In case any of this happens, it is advised to go see a doctor.
3. Is benzoic acid possibly contributing to cancer?
Benzoic acid is not a carcinogen by itself, and thus does not lead to cancer. However, benzoic acid can become carcinogenic, because benzene can be derived from this, and as is common knowledge, benzene is a very significant cancer-causing agent. Now, it is not always possible to avoid benzene totally since it is used in almost all industrial applications. This is why the exposure to benzene needs to be limited greatly so that it does not go to the point of harming someone’s health.
4. Is benzoic acid good for the body?
Although the body should not be overexposed to benzoic acid, there are a lot of very practical benefits that can come from it. Benzoic acid can halt the formation of bacteria and this is applied in topical creams which are used to treat and cure insect bites, redness, swelling, burn wounds, and so on. Due to these antifungal and antibacterial uses, it can also be potentially used to increase digestive functions for people. It is also used as a preservative in many foods.
5. Can benzoic acid harm the environment?
When handling benzoic acid, it is important to know that in controlled quantities, this acid will have no corresponding negative effect on anything. However, in lab environments where there is a greater concentration of this, it is possible for spills and accidents to happen. In that case, it is always important to make sure that everything is gloved and well-shielded. Benzoic acid is not very good for the environment because if left exposed it can potentially react with gases in the air.
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