Welcome to Carbon–Carbon Bond Formation!
In organic chemistry, most of the reactions you have learned so far involve changing one functional group into another (like turning an alcohol into a haloalkane). But what if you want to make the molecule bigger? To do that, you need to form new carbon–carbon (C–C) bonds.
Think of this chapter as learning how to add "bricks" to a building. By creating C–C bonds, chemists can build complex structures like life-saving medicines and high-tech polymers. Don't worry if this seems like a lot of new reactions—they all follow patterns you've seen before. Let’s dive in!
1. Building the Chain: Making Nitriles
One of the best ways to increase the length of a carbon chain is by adding a nitrile group (\(-C \equiv N\)). Because the nitrile group has a carbon atom, adding it automatically makes your carbon chain one atom longer!
A. From Haloalkanes (Nucleophilic Substitution)
If you react a haloalkane with sodium cyanide (\(NaCN\)) or potassium cyanide (\(KCN\)) in ethanol, the cyanide ion (\(CN^-\)) acts as a nucleophile.
The Reaction:
\(CH_3CH_2Br + CN^- \rightarrow CH_3CH_2CN + Br^-\)
(Bromoethane becomes Propanenitrile)
Step-by-Step Process:
1. The lone pair on the Carbon of the \(CN^-\) ion is attracted to the \(\delta+\) Carbon in the C-Halogen bond.
2. The C-Halogen bond breaks (heterolytic fission).
3. A new C–C bond is formed, and the halogen is kicked out.
Quick Tip: Always use ethanol as the solvent. If you use water, you might get an alcohol instead of a nitrile!
B. From Carbonyls (Nucleophilic Addition)
You can also add a carbon atom to aldehydes and ketones. We react them with hydrogen cyanide (\(HCN\)) to form hydroxynitriles.
The Reaction:
\(CH_3CHO + HCN \rightarrow CH_3CH(OH)CN\)
(Ethanal becomes 2-hydroxypropanenitrile)
Safety Note: \(HCN\) is a highly toxic gas. In a school lab, we usually use a mixture of \(NaCN\) and \(H_2SO_4\) to provide the \(HCN\) safely in the reaction flask.
Quick Review:
• Haloalkanes + \(CN^-\) \(\rightarrow\) Nitrile (Substitution)
• Aldehydes/Ketones + \(HCN\) \(\rightarrow\) Hydroxynitrile (Addition)
• Both reactions increase the carbon chain length by one.
Key Takeaway: Nitriles are the "extender cables" of organic chemistry. Whenever you see a synthesis question where the product has more carbons than the starting material, think Cyanide!
2. What can we do with Nitriles?
Once you’ve "grown" your carbon chain by making a nitrile, you can transform that nitrile group into other useful things.
A. Reduction to Amines
You can turn a nitrile into a primary amine by adding hydrogen. This is a reduction reaction.
Reagent/Catalyst: Hydrogen gas (\(H_2\)) with a Nickel (\(Ni\)) catalyst.
Equation:
\(CH_3CH_2CN + 2H_2 \xrightarrow{Ni} CH_3CH_2CH_2NH_2\)
(Propanenitrile becomes Propylamine)
B. Acid Hydrolysis to Carboxylic Acids
If you heat a nitrile with dilute aqueous acid (like \(HCl\)), it reacts with water to form a carboxylic acid.
The Process:
The Nitrogen atom is "kicked out" and replaced by Oxygen atoms from the water.
Equation:
\(CH_3CH_2CN + 2H_2O + HCl \xrightarrow{heat} CH_3CH_2COOH + NH_4Cl\)
(Propanenitrile becomes Propanoic acid)
Did you know? This is a two-for-one deal! You’ve increased the chain length AND created a very reactive carboxylic acid group that you can use for further reactions.
Key Takeaway Summary:
• Nitrile + \(H_2/Ni\) \(\rightarrow\) Amine
• Nitrile + \(H_2O/HCl\) + heat \(\rightarrow\) Carboxylic Acid
3. Friedel–Crafts Reactions: Building on Benzene
Benzene is very stable and doesn't like to react. To form a C–C bond onto a benzene ring, we need a "helper" called a halogen carrier (like \(AlCl_3\) or \(FeCl_3\)). These reactions are called Friedel–Crafts reactions.
A. Alkylation
This adds an alkyl group (like \(-CH_3\) or \(-C_2H_5\)) to the benzene ring.
Reagents: A haloalkane (e.g., \(CH_3Cl\)) and a halogen carrier (\(AlCl_3\)).
Equation:
\(C_6H_6 + CH_3Cl \xrightarrow{AlCl_3} C_6H_5CH_3 + HCl\)
(Benzene becomes Methylbenzene)
B. Acylation
This adds an acyl group (\(-C=O\)) to the benzene ring, creating an aromatic ketone.
Reagents: An acyl chloride (e.g., \(CH_3COCl\)) and a halogen carrier (\(AlCl_3\)).
Equation:
\(C_6H_6 + CH_3COCl \xrightarrow{AlCl_3} C_6H_5COCH_3 + HCl\)
(Benzene becomes Phenylethanone)
Analogy: Imagine the benzene ring is a fortress. The halogen carrier is like a "siege engine" that helps the carbon group break through the stable walls of the benzene ring to form a new bond.
Common Mistake to Avoid: Don't forget that \(HCl\) is always a side-product in these Friedel-Crafts reactions. The Chlorine from the reagent joins with a Hydrogen "kicked off" the benzene ring.
Key Takeaway: Friedel-Crafts reactions allow us to attach carbon chains to benzene. Alkylation adds a simple chain, while Acylation adds a chain with a \(C=O\) "handle" on it.
Final Quick Review Table
To make the chain longer...
1. Aliphatic: Use \(CN^-\) (with haloalkanes) or \(HCN\) (with carbonyls).
2. Aromatic: Use Friedel-Crafts Alkylation or Acylation with an \(AlCl_3\) catalyst.
3. Next Steps: Turn nitriles into amines (use \(H_2/Ni\)) or acids (use \(H_2O/HCl\)).
You've got this! Practice drawing these mechanisms, and you'll start to see how these building blocks fit together in multi-stage synthesis questions.