HYDROBROMIC ACID (HBR): STRUCTURE, PROPERTIES, FORMATION - CHEMISTRY

The hydrobromic acid is an inorganic compound is the aqueous solution of a gas called hydrogen bromide. Its chemical formula is HBr, and it can be considered in different equivalent ways: as a molecular hydride, or a hydrogen halide in water; that is, a hydracid.

In chemical equations it should be written as HBr (ac), thus indicating that it is the hydrobromic acid and not the gas. This acid is one of the strongest known, even more so than hydrochloric acid, HCl. The explanation for this lies in the nature of its covalent bond.

Source: KES47 via Wikipedia

Why is HBr such a strong acid, and even more so dissolved in water? Because the H-Br covalent bond is very weak, due to the poor overlap of the 1s orbitals of H and 4p of Br.

This is not surprising if you look closely at the top image, where clearly the bromine atom (brown) is much larger than the hydrogen atom (white).

Consequently, any disturbance causes the H-Br bond to break, releasing the H ⁺ ion. So, hydrobromic acid is a Brönsted acid, since it transfers protons or hydrogen ions. Its strength is such that it is used in the synthesis of various organobrominated compounds (such as 1-Bromo ethane, CH ₃ CH ₂ Br).

Hydrobromic acid is, after hydroiodic, HI, one of the strongest and most useful hydracids for the digestion of certain solid samples.

Structure of hydrobromic acid

The image shows the structure of H-Br, whose properties and characteristics, even those of a gas, are closely related to its aqueous solutions. That is why there comes a point where there is confusion regarding which of the two compounds is referred to: HBr or HBr (ac).

The structure of HBr (ac) is different from that of HBr, since now the water molecules are solving this diatomic molecule. When it is close enough, the H ⁺ is transferred to a molecule of H ₂ O as indicated by the following chemical equation:

HBr + H ₂ O => Br ^- + H ₃ O ⁺

Thus, the structure of hydrobromic acid consists of Br ^- and H ₃ O ⁺ ions interacting electrostatically. Now, it is a little different than the covalent bond of H-Br.

Its great acidity is due to the fact that the bulky Br ^- anion can barely interact with the H ₃ O ⁺, without being able to prevent it from transferring the H ⁺ to another surrounding chemical species.

Acidity

For example, Cl ^- and F ^- although they do not form covalent bonds with H ₃ O ⁺, they can interact through other intermolecular forces, such as hydrogen bonds (which only F ^- is capable of accepting). Hydrogen bonds F ^- -H-OH ₂ ⁺ "hinder" the donation of H ⁺.

It is for this reason that hydrofluoric acid, HF, is a weaker acid in water than hydrobromic acid; since, the ionic interactions Br ^- H ₃ O ⁺ do not affect the transfer of H ⁺.

However, although water is present in HBr (aq), its behavior is ultimately similar to that of considering a H-Br molecule; that is, an H ⁺ is transferred from HBr or Br ^- H ₃ O ⁺.

Physical and chemical properties

Molecular formula

HBr.

Molecular weight

80.972 g / mol. Note that, as mentioned in the previous section, only the HBr is considered and not the water molecule. If the molecular weight were taken from the formula Br ^- H ₃ O ^{+ it} would have a value of approximately 99 g / mol.

Physical appearance

Colorless or pale yellow liquid, which will depend on the concentration of the dissolved HBr. The more yellow it is, the more concentrated and dangerous it will be.

Odor

Acrid, irritating.

Odor threshold

6.67 mg / m ³.

Density

1.49 g / cm ³ (48% w / w aqueous solution). This value, as well as those for the melting and boiling points, depend on the amount of HBr dissolved in the water.

Melting point

-11 ° C (12 ° F, 393 ° K) (49% w / w aqueous solution).

Boiling point

122 ° C (252 ° F. 393 ° K) at 700 mmHg (47-49% w / w aqueous solution).

Water solubility

-221 g / 100 ml (at 0 ° C).

-204 g / 100 ml (15 ° C).

-130 g / 100 ml (100 ° C).

These values refer to gaseous HBr, not to hydrobromic acid. As can be seen, as the temperature increases, the solubility of HBr decreases; behavior that is natural in gases. Consequently, if concentrated HBr (aq) solutions are required it is better to work with them at low temperatures.

If working at high temperatures, the HBr will escape in the form of gaseous diatomic molecules, so the reactor must be sealed to prevent its leakage.

Vapor density

2.71 (in relation to air = 1).

Acidity pKa

-9.0. This negative constant is indicative of its great acidity strength.

Caloric capacity

29.1 kJ / mol.

Standard molar enthalpy

198.7 kJ / mol (298 K).

Standard molar entropy

-36.3 kJ / mol.

ignition point

Not flammable.

Nomenclature

Its name 'hydrobromic acid' combines two facts: the presence of water, and that bromine has a valence of -1 in the compound. In English it is somewhat more obvious: hydrobromic acid, where the prefix 'hydro' (or hydro) refers to water; although, actually, it can also refer to hydrogen.

Bromine has a valence of -1 because it is bound to a hydrogen atom less electronegative than it; but if it were bound or interacting with oxygen atoms, it can have numerous valences, such as: +2, +3, +5 and +7. With the H it can only adopt a single valence, and therefore the suffix -ico is added to its name.

Whereas HBr (g), hydrogen bromide, is anhydrous; that is, it has no water. Therefore, it is named under other nomenclature standards, corresponding to that of hydrogen halides.

How is it formed?

There are several synthetic methods for preparing hydrobromic acid. Some of them are:

Mix of hydrogen and bromine in water

Without describing the technical details, this acid can be obtained from the direct mixing of hydrogen and bromine in a reactor filled with water.

H ₂ + Br ₂ => HBr

In this way, as the HBr is formed it dissolves in the water; this can drag it in the distillations, so solutions with different concentrations can be extracted. Hydrogen is a gas, and bromine is a dark reddish liquid.

Phosphorus tribromide

In a more elaborate process, sand, hydrated red phosphorus and bromine are mixed. Water traps are placed in ice baths to prevent the HBr from escaping and forming hydrobromic acid instead. The reactions are:

2P + 3Br ₂ => 2PBr ₃

PBr ₃ + 3H ₂ O => 3HBr + H ₃ PO ₃

Sulfur dioxide and bromine

Another way to prepare it is to react bromine with sulfur dioxide in water:

Br ₂ + SO ₂ + 2H ₂ O => 2HBr + H ₂ SO ₄

This is a redox reaction. Br ₂ is reduced, gains electrons, by bonding with hydrogens; While SO ₂ oxidizes, it loses electrons when it forms more covalent bonds with other oxygens, as in sulfuric acid.

Applications

Bromide preparation

Bromide salts can be prepared by reacting HBr (aq) with a metal hydroxide. For example, the production of calcium bromide is considered:

Ca (OH) ₂ + 2HBr => CaBr ₂ + H ₂ O

Another example is for sodium bromide:

NaOH + HBr => NaBr + H ₂ O

Thus, many of the inorganic bromides can be prepared.

Synthesis of alkyl halides

And what about organic bromides? These are organobrominated compounds: RBr or ArBr.

Dehydration of alcohols

The raw material to obtain them can be alcohols. When they are protonated by the acidity of HBr, they form water, which is a good leaving group, and in its place the bulky Br atom is incorporated, which will become covalently bonded with carbon:

ROH + HBr => RBr + H ₂ O

This dehydration is carried out at temperatures above 100 ° C, in order to facilitate the breaking of the R-OH ₂ ⁺ bond.

Addition to alkenes and alkynes

The HBr molecule can be added from its aqueous solution to the double or triple bond of an alkene or alkyne:

R ₂ C = CR ₂ + HBr => RHC-CRBr

RC≡CR + HBr => RHC = CRBr

Several products can be obtained, but under simple conditions, the product is primarily formed where the bromine is linked to a secondary, tertiary or quaternary carbon (Markovnikov's rule).

These halides are involved in the synthesis of other organic compounds, and their range of uses is very extensive. Likewise, some of them may even be used in the synthesis or design of new drugs.

Cleavage of ethers

From the ethers, two alkyl halides can be obtained simultaneously, each carrying one of the two side chains R or R 'of the initial ether RO-R'. Something similar to the dehydration of alcohols happens, but their reaction mechanism is different.

The reaction can be outlined with the following chemical equation:

ROR '+ 2HBr => RBr + R'Br

And water is also released.

Catalyst

Its acidity is such that it can be used as an effective acid catalyst. Instead of adding the Br ^- anion to the molecular structure, it makes way for another molecule to do so.

References

Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10 ^th edition.). Wiley Plus.
Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
Steven A. Hardinger. (2017). Illustrated Glossary of Organic Chemistry: Hydrobromic acid. Recovered from: chem.ucla.edu
Wikipedia. (2018). Hydrobromic acid. Recovered from: en.wikipedia.org
PubChem. (2018). Hydrobromic acid. Recovered from: pubchem.ncbi.nlm.nih.gov
National Institute of Safety and Hygiene at Work. (2011). Hydrogen bromide. Recovered from: insht.es
PrepChem. (2016). Preparation of hydrobromic acid. Recovered from: prepchem.com

HYDROBROMIC ACID (HBR): STRUCTURE, PROPERTIES, FORMATION - CHEMISTRY - 2024