Hydrocarbons Flashcards for NEET — 21 cards

First 8 of 21 Hydrocarbons flashcards

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1. Card 1 of 21

   What is the Wurtz reaction, and what type of alkanes is it best suited to prepare?

   Hint: Uses Na/dry ether.

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   Answer

   Alkyl halides react with sodium metal in dry ether to form alkanes with double the number of carbon atoms. It is best suited for preparing symmetrical alkanes.

2. Card 2 of 21

   Draw or visualize the Newman projection for the staggered conformation of ethane.

   Hint: Newman projections: $60^\circ$ vs $0^\circ$ dihedral angle.

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   Answer

   Staggered Conformation: Bonds of the front carbon (blue) bisect the angles of the back carbon (red).

3. Card 3 of 21

   What are the three steps in the mechanism of halogenation of alkanes (e.g., chlorination of methane)?

   Hint: Free radical substitution.

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   Answer

   1. Initiation: Homolytic cleavage of the halogen molecule (e.g., $Cl_2$) by UV light to form free radicals. 2. Propagation: Halogen radical attacks alkane to form an alkyl radical, which then reacts with another halogen molecule. 3. Termination: Two free radicals combine to end the chain reaction.

4. Card 4 of 21

   How does the boiling point of alkanes vary with increasing chain length and branching?

   Hint: More surface area = higher boiling point.

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   Answer

   Boiling point increases with an increase in chain length (higher molecular mass = stronger van der Waals forces). It decreases with increased branching because the molecule becomes more spherical, reducing the surface area for intermolecular contact.

5. Card 5 of 21

   State Markownikoff's Rule for the addition of hydrogen halides to unsymmetrical alkenes.

   Hint: The rich get richer (H goes to the C with more H's).

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   Answer

   When an unsymmetrical reagent (like $HX$) adds to an unsymmetrical alkene, the negative part of the reagent (e.g., $X^-$) attaches to the carbon atom of the double bond that carries the lesser number of hydrogen atoms.

6. Card 6 of 21

   What is the Peroxide Effect (Kharasch effect), and with which hydrogen halide is it observed?

   Hint: Operates via a free radical mechanism.

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   Answer

   In the presence of organic peroxides, the addition of $HBr$ to unsymmetrical alkenes follows an Anti-Markownikoff pattern (Bromine attaches to the carbon with MORE hydrogens). This effect is ONLY observed with $HBr$, not with $HCl$ or $HI$.

7. Card 7 of 21

   What is the intermediate formed during the electrophilic addition of halogens (like $Br_2$) to alkenes?

   Hint: Not a standard carbocation. It's a three-membered ring with a positive charge on the halogen.

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   Answer

   A cyclic halonium ion intermediate (e.g., cyclic bromonium ion). This intermediate ensures that the subsequent nucleophilic attack happens from the opposite side, leading to anti-addition.

8. Card 8 of 21

   What is the structural difference between primary, secondary, and tertiary carbon atoms in an alkane?

   Hint: Count the carbon neighbors.

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   Answer

   Primary ($1^\circ$): Carbon attached to one other carbon. Secondary ($2^\circ$): Carbon attached to two other carbons. Tertiary ($3^\circ$): Carbon attached to three other carbons.