Question

Write the formulas of the products formed from the reaction of 2 -butene with each of the following substances: (a) \(\mathrm{H}_{2} ;\) (b) \(\mathrm{Cl}_{2} ;\) (c) \(\mathrm{HCl} ;\) (d) \(\mathrm{H}_{2} \mathrm{O}\) (in acid).

Short answer

The products formed from the reaction of 2-butene with the given substances are: (a) butane, (b) 2,3-dichlorobutane, (c) 2-chlorobutane, and (d) 2-butanol.

Step-by-step solution

Hydrogenation of 2-Butene

Reaction (a) is an example of hydrogenation, where molecular hydrogen (\(\mathrm{H}_2\)) is added to the alkene. In this case, hydrogen is added to 2-butene, resulting in butane. This reaction removes the double bond and turns the alkene into an alkane. The general formula for this reaction is \( \mathrm{RCH=CHR} + \mathrm{H}_2 \rightarrow \mathrm{RCH}_2\mathrm{CHR}_2\). Replace \(\mathrm{R}\) with an ethyl group (\(\mathrm{-CH_2CH_3}\)) to get the product for this specific case, which will yield butane (\(\mathrm{C}_4\mathrm{H}_{10}\)).

Halogenation of 2-Butene

Reaction (b) is a halogenation reaction, where a halogen is added across the double bond in the alkene. In 2-butene's case, it reacts with \(\mathrm{Cl}_2\) to form 2,3-dichlorobutane. The general formula is \(\mathrm{RCH=CHR} + \mathrm{X}_2 \rightarrow \mathrm{RCHX-CHXR}\). Replace \(\mathrm{R}\) with an ethyl group (\(\mathrm{-CH_2CH_3}\)) and \(\mathrm{X}\) with a Chlorine atom (\(\mathrm{Cl}\)), and this will yield 2,3-dichlorobutane (\(\mathrm{C}_4\mathrm{H}_8\mathrm{Cl}_2\)).

Hydrohalogenation of 2-Butene

Reaction (c) is a hydrohalogenation reaction, where a hydrogen halide (in this case \(\mathrm{HCl}\)) is added to the alkene. This results in 2-chlorobutane. For this specific case, follow the general equation \(\mathrm{RCH=CHR} + \mathrm{HX} \rightarrow \mathrm{RCHX-CH_2R}\). Again, replace \(\mathrm{R}\) with an ethyl group (\(\mathrm{-CH_2CH_3}\)) and \(\mathrm{X}\) with a Chlorine atom (\(\mathrm{Cl}\)), and the product is 2-chlorobutane (\(\mathrm{C}_4\mathrm{H}_9\mathrm{Cl}\)).

Hydration of 2-Butene

Finally, for reaction (d), 2-butene undergoes hydration in the presence of an acid to form 2-butanol. This is the general reaction \(\mathrm{RCH=CHR} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{RCHOH-CH_2R}\). Replacing \(\mathrm{R}\) with an ethyl group (\(\mathrm{-CH_2CH_3}\)) will yield 2-butanol (\(\mathrm{C}_4\mathrm{H}_9\mathrm{OH}\)).

Key concepts

Hydrogenation

Hydrogenation is a chemical reaction where hydrogen atoms are added to a molecule. In the case of 2-butene, hydrogenation involves the addition of molecular hydrogen (\(\mathrm{H}_2\)) across the double bond. This process converts the alkene, 2-butene, into an alkane, specifically butane (\(\mathrm{C}_4\mathrm{H}_{10}\)).

The double bond in the 2-butene allows it to react with hydrogen, which breaks the bond and results in a saturated hydrocarbon. This reaction is typically catalyzed by metals such as platinum, palladium, or nickel, which help facilitate the addition of hydrogen. Hydrogenation is commonly used in the food industry to convert liquid oils into solid fats, such as margarine.

Overall, the reaction can be summarized as:

• Starting material: 2-butene
  
• Reagent: Molecular hydrogen (\(\mathrm{H}_2\))
  
• Product: Butane (\(\mathrm{C}_4\mathrm{H}_{10}\))
  

Halogenation

In halogenation, a halogen molecule such as chlorine (\(\mathrm{Cl}_2\)) is added across the double bond of an alkene. For 2-butene, this means that the chlorine atoms will attach to the carbon atoms involved in the double bond.

This reaction results in the formation of a dihalogenated compound, specifically, 2,3-dichlorobutane (\(\mathrm{C}_4\mathrm{H}_8\mathrm{Cl}_2\)). The addition of \(\mathrm{Cl}_2\) breaks the original double bond, introducing two chlorine atoms into the molecule, one at each carbon of the former double bond.

Halogenation is an important reaction in organic chemistry because it transforms unsaturated hydrocarbons into more reactive or differently functionalized compounds. It’s often used to create intermediates for further chemical synthesis.

To sum up the halogenation of 2-butene:

• Starting material: 2-butene
  
• Reagent: Chlorine (\(\mathrm{Cl}_2\))
  
• Product: 2,3-dichlorobutane (\(\mathrm{C}_4\mathrm{H}_8\mathrm{Cl}_2\))
  

Hydrohalogenation

Hydrohalogenation involves the addition of hydrogen halides, such as hydrochloric acid (\(\mathrm{HCl}\)), to an alkene like 2-butene. This reaction leads to the formation of an alkyl halide, more specifically, 2-chlorobutane (\(\mathrm{C}_4\mathrm{H}_9\mathrm{Cl}\)).

When 2-butene reacts with \(\mathrm{HCl}\), the double bond's electrons attack the hydrogen of the \(\mathrm{HCl}\), leading to the formation of a carbocation intermediate. Chlorine then bonds to the carbocation, producing the final product. This reaction tends to follow Markovnikov's rule, meaning that the hydrogen atom from \(\mathrm{HCl}\) will attach to the less substituted carbon of the double bond.

Hydrohalogenation is an effective method for adding halogen atoms to unsaturated hydrocarbons and is applicable in producing a variety of halogenated organic compounds.

Here’s a quick breakdown:

• Starting material: 2-butene
  
• Reagent: Hydrochloric acid (\(\mathrm{HCl}\))
  
• Product: 2-chlorobutane (\(\mathrm{C}_4\mathrm{H}_9\mathrm{Cl}\))
  

Hydration

Hydration of alkenes, like 2-butene, involves the addition of water (\(\mathrm{H}_2\mathrm{O}\)) across the double bond, usually in the presence of an acid catalyst, such as sulfuric acid. This reaction results in the formation of an alcohol; for 2-butene, the specific product is 2-butanol (\(\mathrm{C}_4\mathrm{H}_9\mathrm{OH}\)).

The acid catalyst provides a proton, \(\mathrm{H}^+\), which initiates the reaction by forming a carbocation on one of the carbons from the double bond. Water then reacts with this carbocation, yielding an alcohol after a final proton transfer.

Hydration reactions are crucial in industrial settings for the production of alcohols, which are important as solvents and chemical feedstock. In hydration, Markovnikov's rule applies, often directing the hydroxyl group to the more substituted carbon.

Overall summary:

• Starting material: 2-butene
  
• Reagent: Water (\(\mathrm{H}_2\mathrm{O}\)) with an acid catalyst
  
• Product: 2-butanol (\(\mathrm{C}_4\mathrm{H}_9\mathrm{OH}\))