Exercises: Main organic reactions

Propene ($\mathrm{CH_3-CH=CH_2}$) undergoes an electrophilic addition reaction when treated with water in the presence of dilute sulfuric acid as catalyst, following Markovnikov's rule (the proton is added preferentially to the carbon with the greater number of hydrogens).

1. a) Write the formula and name of the starting reagent and of the major organic product obtained in this hydration.
2. b) Justify, with reasoning, why propan-1-ol is not obtained as the major product.

Complete combustion of alcohols produces carbon dioxide and water vapour, releasing a considerable amount of heat energy.

1. a) Write and balance, using whole-number stoichiometric coefficients, the chemical equation for the complete combustion of ethanol ($\mathrm{CH_3-CH_2OH}$).
2. b) Calculate the volume of carbon dioxide, measured at $25\,^{\circ}\text{C}$ and $1.0\,\text{atm}$, released when $23.0\,\text{g}$ of ethanol is burned completely. Data: relative atomic masses: $\mathrm{C}=12.0$; $\mathrm{H}=1.0$; $\mathrm{O}=16.0$; gas constant $R = 0.082\,\text{atm}\times\text{L}\times\text{mol}^{-1}\times\text{K}^{-1}$.

Alkyl halides can undergo selective transformations depending on the nature of the reagent and the solvent used.

1. a) Write the balanced equation for the nucleophilic substitution reaction between 1-bromopropane ($\mathrm{CH_3-CH_2-CH_2Br}$) and a dilute aqueous solution of sodium hydroxide at moderate temperature, naming the organic product formed.
2. b) State the semi-structural formula and the name of the product that would be obtained if the propene formed in a side reaction were subjected to catalytic hydrogenation with $\mathrm{H_2}$ in the presence of nickel.

The chemical behaviour of a primary alcohol towards a strong oxidising agent such as potassium dichromate in acidic medium depends on the experimental conditions used. Observe the following transformation scheme

1. a) Identify, with justification, the semi-structural formulae and systematic names of the organic products $\mathrm{A}$ and $\mathrm{B}$.
2. b) Justify why immediate distillation of the mixture allows compound $\mathrm{A}$ to be isolated selectively, whereas prolonged reflux with excess oxidant leads to $\mathrm{B}$.

In a closed reactor of $1.00\,\text{L}$ at a constant temperature of $25\,^{\circ}\text{C}$, $2.00\,\text{mol}$ of acetic acid (ethanoic acid) and $2.00\,\text{mol}$ of ethanol are mixed in the presence of a catalytic amount of concentrated sulfuric acid.

1. a) Write the esterification reaction, giving the names of the ester and of the inorganic by-product formed.
2. b) Given that the dimensionless equilibrium constant for this esterification in solution is $K_c = 4.00$ at this temperature, calculate the amount of ester (in moles) present at equilibrium and the percentage yield of the process. Initially there is no ester or water. Assume an ideal medium of constant volume prepared with anhydrous reagents and, if necessary, an inert solvent; include the water formed in the equilibrium expression.

Basic hydrolysis, or saponification, of ethyl acetate, $\mathrm{CH_3-COO-CH_2-CH_3}$, consumes a strong base. a) Write the molecular equation for its reaction with aqueous sodium hydroxide solution and name the two organic products obtained, one of them in the form of a salt. b) $17.6\,\mathrm{g}$ of the ester is treated with $250\,\mathrm{mL}$ of $\mathrm{NaOH}$ $1.00\,\mathrm{mol}\times\mathrm{L}^{-1}$. Identify the limiting reagent and calculate the mass of sodium acetate isolated if the yield with respect to the stoichiometric maximum is $85.0\,\%$. Data: relative atomic masses, C = 12.0; H = 1.0; O = 16.0; Na = 23.0.

It is desired to synthesise propanone in the laboratory by a two-step sequence starting from propene. In step 1, propene is hydrated in the presence of aqueous sulfuric acid following Markovnikov's rule, giving a secondary alcohol as the major product. In step 2, this alcohol is purified and oxidised with potassium dichromate in acidic medium to obtain propanone.

1. a) Write the equations for both steps, classifying each reaction, with justification, according to the type of organic process (addition, substitution, elimination, oxidation-reduction).
2. b) If you start with $84.0\,\text{g}$ of propene and step 1 has a yield of $75.0\,\%$ while step 2 has a yield of $80.0\,\%$, calculate the final mass of pure propanone obtained. Atomic masses: $\mathrm{C}=12.0$; $\mathrm{H}=1.0$; $\mathrm{O}=16.0$.

A route of three consecutive steps is designed to synthesise ethyl acetate from ethene ($\mathrm{CH_2=CH_2}$):

Step 1: Acid hydration of ethene to obtain ethanol with a yield of $80.0\,\%$.

Step 2: Vigorous oxidation under reflux of half of the moles of ethanol obtained in step 1 using excess $\mathrm{KMnO_4/H^+}$ to acetic acid, with a yield of $90.0\,\%$.

Step 3: Condensation (Fischer esterification) between the acetic acid obtained in step 2 and the other half of the ethanol set aside from step 1.

1. a) Write the equations for the three steps, stating the usual conditions and catalysts.
2. b) If the process is started with $112\,\text{g}$ of ethene and the yield of the final esterification (step 3) with respect to the limiting reagent is $70.0\,\%$, calculate the mass of ethyl acetate obtained. Atomic masses: $\mathrm{C}=12.0$; $\mathrm{H}=1.0$; $\mathrm{O}=16.0$.

A historical organic analysis report describes four unlabelled bottles ($\mathrm{A}$, $\mathrm{B}$, $\mathrm{C}$ and $\mathrm{D}$), each of which contains one of the following pure liquid substances: propanal, propanone, propanoic acid and hex-1-ene. To identify them, three independent chemical tests were recorded, which are analysed as data without reproducing them:

1. Addition of a reddish solution of bromine in carbon tetrachloride: only substance $\mathrm{C}$ decolourises the solution quickly and immediately without evolution of gas.

2. Addition of saturated aqueous sodium hydrogencarbonate solution ($\mathrm{NaHCO_3}$): only substance $\mathrm{D}$ produces clearly visible effervescence.

3. Test with Tollens' reagent (ammoniacal solution of $\mathrm{Ag^+}$): substance $\mathrm{A}$ forms a silver mirror of metallic silver on the walls of the tube when gently heated, whereas $\mathrm{B}$ produces no visible change.

1. a) Assign, with justification, each substance to the letters $\mathrm{A}$, $\mathrm{B}$, $\mathrm{C}$ and $\mathrm{D}$.
2. b) Write the chemical equations corresponding to the positive tests described for $\mathrm{C}$ and $\mathrm{D}$.