Calculation of mass fractions and derivation of the empirical formula
The process of structural elucidation begins with analyzing the composition of the final product. If hydrogen chloride ($\text{HCl}$) was involved in the reaction and only the mass fractions of carbon and hydrogen are known, the halogen content must be calculated. To do this, the known fractions are summed (e.g., $42.6\%$ $\text{C}$ and $7.1\%$ $\text{H}$, giving $49.7\%$) and the result is subtracted from $100\%$. The remaining $50.3\%$ corresponds to chlorine.
Next, the molar ratio of the elements in the substance is determined. To do this, the mass fraction of each element is divided by its relative atomic mass ($\text{Ar}$):
- Carbon ($\text{Ar}=12$): $42.6 / 12 = 3.55$
- Hydrogen ($\text{Ar}=1$): $7.1 / 1 = 7.1$
- Chlorine ($\text{Ar}=35.5$): $50.3 / 35.5 = 1.42$
To obtain the subscripts for the simplest formula, all obtained values are divided by the smallest of them (in this case, $1.42$). This yields a ratio of $\text{C}=2.5$, $\text{H}=5$, and $\text{Cl}=1$. Since subscripts must be integers, the entire series is multiplied by two. The final molecular formula of the hydrochlorination product is $\text{C}_5\text{H}_{10}\text{Cl}_2$.
Product structure elucidation
The derived formula $\text{C}_5\text{H}_{10}\text{Cl}_2$ corresponds to a pentane dichloro derivative. The presence of exactly two halogen atoms indicates that two molecules of hydrogen chloride added to the starting molecule. This means that the starting hydrocarbon had a high degree of unsaturation—it belonged to the class of alkynes or dienes with the general formula $\text{C}_5\text{H}_8$.
A key factor for accurately determining the structure is the physical property of the product—its symmetry, as stated in the synthesis conditions. For a five-carbon skeleton, the only arrangement of two chlorine atoms that allows the molecule to retain full spatial symmetry is their addition to the central carbon atom. Thus, the reaction product is exclusively 3,3-dichloropentane ($\text{CH}_3-\text{CH}_2-\text{CCl}_2-\text{CH}_2-\text{CH}_3$).
Reconstruction of the initial unsaturated hydrocarbon
Based on the structure of the final substance, the structure of the initial reagent can be reconstructed. In order for the halogens to end up on the third carbon atom during hydrochlorination, the multiple bond in the starting chain must be located accordingly.
The substance meeting all the requirements is pent-2-yne ($\text{CH}_3-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_3$). The hydrochlorination of this alkyne obeys Markovnikov's rule. Although the pent-2-yne molecule itself is almost symmetrical in terms of electron density distribution, the successive addition of two $\text{HCl}$ molecules leads to the formation of strictly symmetrical 3,3-dichloropentane:
$\text{CH}_3-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_3 + 2\text{HCl} \rightarrow \text{CH}_3-\text{CH}_2-\text{CCl}_2-\text{CH}_2-\text{CH}_3$
Briefly: answers to topical questions
What reagents are involved in the reaction to produce 3,3-dichloropentane?
Pent-2-yne and hydrogen chloride are involved in the reaction to produce 3,3-dichloropentane. The process is a hydrochlorination reaction proceeding in accordance with Markovnikov's rule.
Reacting substances:
- Pent-2-yne — the starting unsaturated hydrocarbon with the structure $\text{CH}_3-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_3$.
- Hydrogen chloride ($\text{HCl}$) — the reagent that adds to the alkyne in an amount of two molecules.
As a result of the interaction of these substances, a symmetrical product $\text{C}_5\text{H}_{10}\text{Cl}_2$ — 3,3-dichloropentane — is formed.
Symmetry dictates the center: in an odd pentane chain (5 carbon atoms), a symmetrical position of the halogens is only possible at the strictly central, third carbon atom.
Class and exam questions
How to determine the mass fraction of chlorine if it is not given directly?
If it is known that the reaction proceeded with hydrogen chloride, you must add the known mass fractions of carbon and hydrogen, and then subtract the resulting sum from 100%.
Why are intermediate subscripts multiplied by two when calculating the formula?
After division by the smallest number, the carbon index may turn out to be fractional (e.g., 2.5). To bring the formula to realistic integer numbers of atoms, the entire series is multiplied by 2.
What does the addition of two HCl molecules indicate?
The presence of two halogen atoms in the product means that the starting hydrocarbon had two π-bonds, which is characteristic of alkynes or diene hydrocarbons.
What else to study on this topic
- Mechanism of electrophilic addition (hydrochlorination) to alkynes
- Application of Markovnikov's rule to nearly symmetrical internal alkynes
- Isomerism and nomenclature of pentane dichloro derivatives
- Comparative reactivity of alkynes and dienes in reactions with hydrogen halides
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