Harvard CHEM 17 - Organic Chemistry I: Structure & Reactivity Flashcards

Question 1

What is the hybridization of the carbon atoms in methane ($CH_4$), ethene ($C_2H_4$), and ethyne ($C_2H_2$)?

Accepted Answer

Methane: $sp^3$ (tetrahedral); Ethene: $sp^2$ (trigonal planar); Ethyne: $sp$ (linear).

Question 2

Describe the key characteristics of a sigma ($\sigma$) bond and a pi ($\pi$) bond.

Accepted Answer

A $\sigma$ bond results from head-on overlap of atomic orbitals (s-s, s-p, p-p), allowing free rotation. A $\pi$ bond results from side-by-side overlap of unhybridized p-orbitals, restricting rotation and forming above and below the internuclear axis.

Question 3

What are the primary rules for drawing valid resonance structures?

Accepted Answer

1. Only electrons (lone pairs, $\pi$ electrons) move, not atoms. 2. The total number of electrons and overall formal charge must be conserved. 3. Atoms in the second row (C, N, O, F) cannot have more than 8 valence electrons (octet rule).

Question 4

Explain the concept of formal charge and how it is calculated.

Accepted Answer

Formal charge is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms. It's calculated as: Formal Charge = (Valence electrons) - (Non-bonding electrons) - (1/2 Bonding electrons).

Question 5

Define constitutional isomers and provide an example.

Accepted Answer

Constitutional isomers (or structural isomers) are compounds with the same molecular formula but different connectivity of atoms. For example, n-butane and isobutane ($C_4H_{10}$) are constitutional isomers.

Question 6

What is a chiral center (stereocenter) in organic chemistry?

Accepted Answer

A chiral center is typically a carbon atom bonded to four different groups. The presence of a chiral center makes a molecule chiral, meaning it is non-superimposable on its mirror image.

Question 7

How do you assign R/S configuration to a chiral center?

Accepted Answer

1. Assign priorities (1-4) to the four groups based on atomic number (Cahn-Ingold-Prelog rules). 2. Orient the molecule so the lowest priority group (4) points away from the viewer. 3. Trace a path from priority 1 to 2 to 3. Clockwise is R (Rectus), counter-clockwise is S (Sinister).

Question 8

Differentiate between enantiomers and diastereomers.

Accepted Answer

Enantiomers are stereoisomers that are non-superimposable mirror images of each other (e.g., R and S configurations of a single chiral center). Diastereomers are stereoisomers that are not mirror images of each other (e.g., cis/trans isomers, or compounds with multiple chiral centers where not all are inverted).

Question 9

What is a meso compound?

Accepted Answer

A meso compound is an achiral compound that contains chiral centers. It possesses an internal plane of symmetry or a center of inversion, making it optically inactive despite having chiral centers.

Question 10

Provide the IUPAC name for the compound $CH_3CH_2CH(CH_3)CH_2CH_3$.

Accepted Answer

3-methylpentane.

Question 11

Draw the Newman projection for the anti and gauche conformations of butane, indicating which is more stable.

Accepted Answer

Anti conformation: The two methyl groups are 180° apart (most stable due to minimal steric strain). Gauche conformation: The two methyl groups are 60° apart (less stable than anti due to gauche butane interaction/steric strain).

Question 12

Describe the two chair conformations of cyclohexane and the terms 'axial' and 'equatorial'.

Accepted Answer

Cyclohexane rapidly interconverts between two chair conformations via a 'chair flip'. Axial substituents point straight up or down, perpendicular to the ring. Equatorial substituents point out from the ring, roughly parallel to the ring plane. Larger groups prefer the equatorial position due to reduced 1,3-diaxial interactions.

Question 13

What is the ARIO rule for predicting acidity?

Accepted Answer

ARIO is an acronym for factors that stabilize a conjugate base, thereby increasing the acidity of its parent acid: **A**tom (electronegativity, size of the atom bearing the negative charge), **R**esonance (delocalization of the negative charge), **I**nduction (electron-withdrawing/donating groups), **O**rbital (hybridization of the atom bearing the charge).

Question 14

How does a lower $pKa$ value relate to acid strength?

Accepted Answer

A lower $pKa$ value indicates a stronger acid. For example, an acid with $pKa = 2$ is significantly stronger than an acid with $pKa = 5$, meaning it dissociates more readily.

Question 15

Describe the $S_N2$ reaction mechanism, including its stereochemistry and substrate preference.

Accepted Answer

$S_N2$ (Substitution Nucleophilic Bimolecular): A concerted, one-step mechanism where the nucleophile attacks from the backside, leading to inversion of configuration at the chiral center. Favored by methyl > primary > secondary substrates; tertiary substrates do not react via $S_N2$. Requires a strong nucleophile and aprotic solvent.

Question 16

Describe the $S_N1$ reaction mechanism, including its stereochemistry and substrate preference.

Accepted Answer

$S_N1$ (Substitution Nucleophilic Unimolecular): A two-step mechanism involving a carbocation intermediate. Leads to racemization (or partial racemization) if the carbocation is chiral. Favored by tertiary > secondary substrates (due to carbocation stability); primary/methyl do not react via $S_N1$. Requires a weak nucleophile and protic solvent.

Question 17

What is Zaitsev's Rule in elimination reactions?

Accepted Answer

Zaitsev's Rule states that in an elimination reaction, the major product will be the more substituted alkene (the one with the most alkyl groups attached to the double bond carbons). This is generally the thermodynamically more stable product.

Question 18

Describe the $E2$ reaction mechanism, including its stereochemistry and substrate preference.

Accepted Answer

$E2$ (Elimination Bimolecular): A concerted, one-step mechanism where a strong base removes a proton and the leaving group departs simultaneously. Requires anti-periplanar geometry of the $\beta$-hydrogen and leaving group. Favored by tertiary > secondary > primary substrates (with strong, bulky bases often leading to Hoffmann product).

Question 19

Describe the $E1$ reaction mechanism, including its stereochemistry and substrate preference.

Accepted Answer

$E1$ (Elimination Unimolecular): A two-step mechanism involving a carbocation intermediate, similar to $S_N1$. The base removes a proton from an adjacent carbon after carbocation formation. Favored by tertiary > secondary substrates. Leads to the Zaitsev product.

Question 20

How do solvent choices (protic vs. aprotic) influence $S_N1/S_N2$ reactions?

Accepted Answer

Protic solvents (e.g., water, alcohols) stabilize carbocations and solvate nucleophiles, favoring $S_N1$. Aprotic solvents (e.g., DMSO, acetone, acetonitrile) do not solvate nucleophiles strongly, making them more reactive and favoring $S_N2$.

Question 21

What factors favor an $E2$ reaction over an $S_N2$ reaction when both are possible?

Accepted Answer

Strong, bulky bases (e.g., t-BuOK) favor $E2$. Higher temperatures also favor elimination reactions over substitution. Increased steric hindrance around the electrophilic carbon also favors $E2$ by making nucleophilic attack more difficult.

Question 22

What is the IUPAC name for the compound $CH_3CH_2C \equiv CCH_3$?

Accepted Answer

Pent-2-yne.

Harvard CHEM 17 - Organic Chemistry I: Structure & Reactivity

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