MCAT Organic Chemistry UPDATED
Questions and CORRECT Answers
Electrostatic Forces - CORRECT ANSWER- The attractive force between electrons and the
nuclei that is responsible for all molecular bonds.
Coordinate Covalent Bond - CORRECT ANSWER- One nucleus donates both of the
electrons in the bond.
Hybridization - CORRECT ANSWER- sp - 180° - linear
sp2 - 120° - trigonal planar
sp3 - 109.5° - tetrahedral, pyramidal, or bent
dsp3 - 90°, 120° - trigonal-bipyramidal, seesaw, t-shaped, linear
d2sp3 - 90° - octahedral, square pyramidal, square planar
When one or more shape is possible, it is determined by the number and position of the lone
pairs of electrons.
Instantaneous Dipole Moment - CORRECT ANSWER- Exists in an otherwise nonpolar
molecule. Occurs because the electrons in a bond move about the orbital and at any moment
may not be distributed exactly between the two bonding atoms, even when the atoms are
identical.
London Dispersion Forces - CORRECT ANSWER- Occur between two instantaneous
dipoles; the weakest dipole-dipole force (vs. hydrogen bonds which are the strongest dipole-
dipole forces).
Zwitterion - CORRECT ANSWER- A neutral molecule with a positive and negative electrical
charge at different locations within the molecule at a pH of 7.
,Ex: amino acids -> amine group deprotonates the carboxylic acid group.
Isomers - CORRECT ANSWER- Molecules that have the same molecular formula but are
different compounds.
Conformational Isomers (Conformers) - CORRECT ANSWER- Not true isomers - different
spatial orientations of the same molecule. At low temperatures, the anti-conformation is the
most common.
Structural Isomers - CORRECT ANSWER- Have the same molecular formula but different
bond-to-bond connectivity; simplest form of isomer.
Stereoisomers - CORRECT ANSWER- Two molecules with the same molecular formula and
same bond-to-bond connectivity that are not the same compound. Like conformational
isomers but contain at least one *chiral/stereo center (C bonded to 4 different substituents).
2 types:
- Enantiomers
- Diastereomers
Enantiomers - CORRECT ANSWER- Stereoisomers that have opposite absolute
configurations at EACH chiral center (mirror images). Cannot be separated by physical
means.
When in equal concentrations -> racemic mix.
Resolution = the separation of enantiomers.
Same physical and chemical characteristics except in 2 cases:
1) Reactions with other chiral compounds.
2) Reactions with polarized light.
Diastereomers - CORRECT ANSWER- Stereoisomers that are not mirror images (are not the
same compound). Can be separated by physical means (crystallization).
, Geometric isomer: special type of diastereomer, have different physical properties.
- Cis: has dipole moment -> strong intermolecular forces so has high boiling point; lower
symmetry does not form crystals as easily so has lower melting point; has steric hindrance so
has higher heats of combustion.
- Trans: no dipole moment - lower boiling point, higher melting point, lower heats of
combustion.
--> better to use E (Opposite) and Z (same)
Epimer - CORRECT ANSWER- Diastereomers that differ at only one chiral center.
Anomers - CORRECT ANSWER- Two diastereomers formed from a ring closure at an
epimeric C.
Anomeric carbon: the chiral carbon of the anomer.
Ex: glucose - anomeric C determines if it's α-glucose or β-glucose.
Meso Compounds - CORRECT ANSWER- Optically inactive (achiral) molecule with two
chiral centers; have a plane of symmetry between their centers which divides the molecule
into halves that are mirror images of each other.
Absolute Configuration - CORRECT ANSWER- The only way to absolutely describe a chiral
molecule.
R = rectus: right (clockwise)
S = sinister: left (counterclockwise)
*for when H (or any other 4th priority substituent) is on DASHES.
Mirror images of chiral molecules always have opposite absolute configurations.
Questions and CORRECT Answers
Electrostatic Forces - CORRECT ANSWER- The attractive force between electrons and the
nuclei that is responsible for all molecular bonds.
Coordinate Covalent Bond - CORRECT ANSWER- One nucleus donates both of the
electrons in the bond.
Hybridization - CORRECT ANSWER- sp - 180° - linear
sp2 - 120° - trigonal planar
sp3 - 109.5° - tetrahedral, pyramidal, or bent
dsp3 - 90°, 120° - trigonal-bipyramidal, seesaw, t-shaped, linear
d2sp3 - 90° - octahedral, square pyramidal, square planar
When one or more shape is possible, it is determined by the number and position of the lone
pairs of electrons.
Instantaneous Dipole Moment - CORRECT ANSWER- Exists in an otherwise nonpolar
molecule. Occurs because the electrons in a bond move about the orbital and at any moment
may not be distributed exactly between the two bonding atoms, even when the atoms are
identical.
London Dispersion Forces - CORRECT ANSWER- Occur between two instantaneous
dipoles; the weakest dipole-dipole force (vs. hydrogen bonds which are the strongest dipole-
dipole forces).
Zwitterion - CORRECT ANSWER- A neutral molecule with a positive and negative electrical
charge at different locations within the molecule at a pH of 7.
,Ex: amino acids -> amine group deprotonates the carboxylic acid group.
Isomers - CORRECT ANSWER- Molecules that have the same molecular formula but are
different compounds.
Conformational Isomers (Conformers) - CORRECT ANSWER- Not true isomers - different
spatial orientations of the same molecule. At low temperatures, the anti-conformation is the
most common.
Structural Isomers - CORRECT ANSWER- Have the same molecular formula but different
bond-to-bond connectivity; simplest form of isomer.
Stereoisomers - CORRECT ANSWER- Two molecules with the same molecular formula and
same bond-to-bond connectivity that are not the same compound. Like conformational
isomers but contain at least one *chiral/stereo center (C bonded to 4 different substituents).
2 types:
- Enantiomers
- Diastereomers
Enantiomers - CORRECT ANSWER- Stereoisomers that have opposite absolute
configurations at EACH chiral center (mirror images). Cannot be separated by physical
means.
When in equal concentrations -> racemic mix.
Resolution = the separation of enantiomers.
Same physical and chemical characteristics except in 2 cases:
1) Reactions with other chiral compounds.
2) Reactions with polarized light.
Diastereomers - CORRECT ANSWER- Stereoisomers that are not mirror images (are not the
same compound). Can be separated by physical means (crystallization).
, Geometric isomer: special type of diastereomer, have different physical properties.
- Cis: has dipole moment -> strong intermolecular forces so has high boiling point; lower
symmetry does not form crystals as easily so has lower melting point; has steric hindrance so
has higher heats of combustion.
- Trans: no dipole moment - lower boiling point, higher melting point, lower heats of
combustion.
--> better to use E (Opposite) and Z (same)
Epimer - CORRECT ANSWER- Diastereomers that differ at only one chiral center.
Anomers - CORRECT ANSWER- Two diastereomers formed from a ring closure at an
epimeric C.
Anomeric carbon: the chiral carbon of the anomer.
Ex: glucose - anomeric C determines if it's α-glucose or β-glucose.
Meso Compounds - CORRECT ANSWER- Optically inactive (achiral) molecule with two
chiral centers; have a plane of symmetry between their centers which divides the molecule
into halves that are mirror images of each other.
Absolute Configuration - CORRECT ANSWER- The only way to absolutely describe a chiral
molecule.
R = rectus: right (clockwise)
S = sinister: left (counterclockwise)
*for when H (or any other 4th priority substituent) is on DASHES.
Mirror images of chiral molecules always have opposite absolute configurations.
