O. Chem 1 Flashcards
Valence Shell Electron Pair Repulsion Theory (VSEPR)
Predicts which shape molecules will take due to the repulsion of lone pairs of electrons
Bonding
Pi bonds: prevent rotation, weaker, less stable, more reactive
Sigma bonds: allow for rotation, stronger, more stable, less reactive
Shape and bond angle are determined by:
1) hybridization
- sp = linear (180)
- sp2 = trigonal planar or bent (120)
- sp3 = tetrahedral, trigonal pyramidal, or bent (109.5)
- sp3d = trigonal bipyramidal, seesaw, T-shaped, or linear (90/120 or 180)
- sp3d2 = octahedral, square pyramidal, or square planar (90)
Shape and bond angle are determined by:
2) Lone pairs of electrons
The presence or absence of unpaired electrons determines the exact shape from among choices.
Know pictures!!!!
Dipole moment
U= &d
&– charge
d- distance between charges
Heat of combustion
When molecules are combusted, all the bonds are broken and then reformed via a radical reaction.
THE LESS STABLE (higher energy) THE BOND THE GREATER WILL BE THE HEAT OF COMBUSTION
The more stable to bond the lower the heat of combustion
Coordinate covalent bonds
A covalent bond in which both electron shared are donated by one atom.
If molecule doesn’t have a lone pair, unlikely to form coordinate covalent bond.
Octet Rule Exeptions
1) Hydrogen and helium are stable with only two electrons in the valence shells
2) Boron an beryllium a re stable with only six electrons in their valence shells
3) Atoms from the third, fourth, fifth period Or higher can accept more than eight electrons. PCl5, SF6, PO4 3-, SO4 2-
Formal charge
FC=valence - assigned
Assigned– lone pair electrons and .5 bonding electrons
Aromaticity:
Huckel’s Rule
4n+2 pi electrons
2, 6, 10, 14…
IUPAC nomenclature
meth-, eth-, prop-, but-, pent-
sec- attachment touching 2 other carbons
iso- branching (away from branch)
- tert- three branching carbons from one
IUPAC Rules
1) Find the longest carbon chain, if there is a tie, the one with most substituents is the parent chain
2) The terminal carbon closest to the substituent is #1. If tie look at 2nd subst.
3) Order substituents alphabetically and give each one a number to match the carbon to which it is attached
4) if more than one of the same substituent is present, use the prefixes di, tri, tetra (2,2-dimethylbutane)
5) Hyphens are placed before and after substituent numbers, but NOT between standard prefixes.
6) Do not consider prefix when alphabetizing substituents if: 1) it represents a number (di, tri) or 2) it includes a hyphen (sec-, tert-). Do alphabetize other prefixes (isopropyl)
Isomers
The MAXIMUM number of optically active stereoisomers for any compound = 2^n, where n is the number of CHIRAL CENTERS
Conformational Isomers
NOT TRUE ISOMERS. It is the same molecule, but twisted or rotated around its bonds, these are considered conformers, NOT isomers.
Structural Isomers
Same formula, different bond-to-bond connectivity
3) Stereoisomers
Same formula, same bond-to-bond connectivity, but different 3-D arrangement of their substituents. There are two categories of stereoisomers: enantiomers and diasteriomers
3a. Enantiomers
Two molecules with the same molecular formula and the same bond-to-bond connectivity that are NON-IDENTICAL, NON-SUPERIMPOSABLE MIRROR IMAGES. They contain at least ONE chiral center.
CHIRALITY
Any atom attached to four DIFFERENT substituents must be chiral. Any atom with less can’t be chiral.
ABSOLUTE CONFIGURATION (R and S): How two enantiomers are distinguished.
-Assign priority to all four substituents based on molecular weight, with higher molecular weight atoms receiving higher priority.
-Rotate the LOWEST priority substituent to the back (into the page). If the priority of the remaining three substituents increases CLOCKWISE it is R, if they increase COUNTERCLOCKWISE it is S.
Rotation of Polar-polarized Light
Enantiomers rotate plane polarized light. R and S enantiomers rotate this light to the same degree but in opposite directions. R can rotate light in CW, so can S.
- If compound rotates light CW, it is called (+) or d (dextrorotary)
- If compound rotate light CCW, it is called (-) or l (levorotary)
3b. Diastereomers
Two molecules with the same molecular formula and same bond-to-bond connectivity that are NON-IDENTICAL AND ARE NOT MIRROR IMAGES. There are three kind of diasteromers: geometric isomers, epimers, and anomers.
3b1. Geometric isomers
Cis/trans. Cis-same side; trans-opposite side.
- E/Z convention: prioritize the two consituents on each carbon by molecular weight. If the two higher priority substituents are on the same side = Z. If the two higher priority substituents are on opposite sides = E.
- Cis isomers usually have dipole moment, trans do not.
- Cis isomers often experience steric hinderance, resulting in a higher energy molecule, but trans do NOT.
3b2. Epimers
Diastereomers that differ at only one chiral center. Many pairs of carbohydrates are epimers (glucose and galactose)
3bc. Anomers
Molecules that differ only in their spatial orientation at the anomeric carbon of a ring structure. If the anomeric OH/OR group at the CH2OH group are on the same side of the ring = BETA ,if they are on opposite sides = ALPHA
4) MESO COMPOUNDS
Molecules with two or more chiral centers that contain a plane of symmetry. This symmetry cancels out their optical activity.
-To recognize them, look for a center of symmetry. However, NOT ALL MOLECULES WITH A PLANE OF SYMMETRY ARE MESO. Check for chiral centers ACROSS the plane of symmetry (not between two molecules, a meso compound is ONE molecule)
