Molecular Structure Flashcards
Lewis Dot Structures
- Find the total number of valence electrons for all atoms in the molecule.
- Use one pair of electons to form one bond between each atom.
- Arrange the remaining electrons around the atoms to satisfy the duet rule for hydrogen and the octet rule for other atoms.
Exceptions: Sometimes atoms within a molecule break the octet rule. Boron and Beryllium do not contain a full octet. Atoms from the third period or higher may have an expanded octet due to vacant d orbitals available for hybridization.
Formal Charge
Valence# - (Cont. bond # + Lone e-s)
Hydrocarbons
Alkane
- Acyclic C<em>n</em>H2n+2
- Cyclic CnH2n
Alkene
Alkyne
Alcohol
Ether
Amine
Carboxylic Acid
Aldehyde
Ketone
Ester
Amide
Prefixes for Organic Nomeclature
Prefix Number of Carbons
Meth- 1
Eth- 2
Prop- 3
But- 4
Pent- 5
Hex- 6
Hept- 7
Oct- 8
Non- 9
Dec- 10
Common Substituents Group
Nomenclature Rules
- The longest carbon chain with the most substituents determines the base names.
- The end carbon closest to a carbon with a substituent is always the first carbon.
- Any substituent is given the same number as its carbon.
- If the same substituent is used more that once, use the prefixes di-, tri-, tetra, and so on.
- Order the substituents alphabetically.
- The ending of the molecules name is based on the most important functional group in the molecule based on IUPAC priority:
- Carboxylic acids
- Aldehydes
- Ketones
- Alcohols
- Alkynes
- Alkenes
- Alkanes
Newman Projections
Hybridization and Molecular Shape
In order o figure out the type of hyrid orbital formed by an atom on the MCAT, simply count the number of sigma bonds and lone pairs of electons on that atom. Mtch this to the sum of the superscripts in a hybrid name.
*Remember, one orbital in the s subshell must be first.
s=1
p=3
d=5
Chirality
A carbon is chiral when it is bonded to four different substituents.
In order to determine the configuration of a given molecule, number the atoms attached to the chiral center carbon with 1 going to the atom with the largest atomic weight.
R is clockwise
S is counterclockwise
Optically Inactive
Optically inactive compounds may be compounds without a chiral center or molecules with internal mirror planes (meso compounds).
Optically Active
Chiral molecules can be separated from their mirror images by chemicals. The result is a compound containing molecules with no mirror images in the compound. When plane-polarized light is projected through such a compound, the orientation of its electric field is rotated. Clockwise rotation is + or d. Counerclockwise is - or l.
Stereoisomers
Two molecules with the same molecular formula and the same bond-to-bond connectivity are mirror images of each other, but not the same compound. There are two types:
- Enantiomers
- Diastereomers
Enantiomers
Enantiomers have the same molecular formula, have the same bond-to-bond connectivity, are mirror images of each other, but are not the same molecule. They have opposite absolute configurations at each chiral carbon (mirror images).
Equal concentration mixtures of enantiomers is call a racemic mixture. They are optically inactive.
Diastereomers
Diastereomers have the same molecular formula, have he same bond-to-bond connectivity, are NOT mirror images to each other, and are NOT the same compound.
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Cis-isomers have similar substituents on opposing carbons on the same-side.
- cis molecule have a dipole moment
- due to dipole moment, cis molecules have stronger intermolecular forces leading to higher boiling points.
- due to their lower symmetry, cis molecules do not form crystals readily, thus having lower melting points.
- Steric hindrance produces higher energy levels resulting in higher heat of combustion.
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Trans-isomers have similar substituents on opposing carbons on the opposite-side.
- trans molecule do not have a dipole moment.
- readily form crystals, thus having a higher melting point.
Tri- and Tetra- Substituted Diasteromers
Isomer Summary
Isomers are different compounds that have the same molecular formula.
Constitutional Isomers, When the group of atoms that make up the molecules of different isomers are bonded together in fundamentally different ways.
