Mcat Organic Chemistry Lecture 1

Front 1

Valence

Back 1

number of bonds an atom usually forms

C = 4, tetravalent
N = 3, trivalent
P = 3, trivalent
O = 2, divalent
S = 2, divalent
H & halogens = 1, monovalent

Front 2

Formal charge

Back 2

number of electrons in the isolated atom, minus # of electrons assigned to the atom in lewis structure

sum of formal charges for each atom in molecule or ion represents total charge on molecule or ion

Front 3

Fischer projection

Back 3

vertical lines are assumed to be oriented into the page

horizontal lines are assume to be oriented out of page

Front 4

Newman projection

Back 4

view straight down axis of 1 of sigma-bonds

both intersecting lines and large circle are assumed to be C atoms

Front 5

Index of Hydrogen Deficiency AKA degree of unsaturation

Back 5

# of pairs of H a compound requires in order to become a saturated alkane

saturated alkane contains (2n + 2) # of H, where n = # C

A double bond and ring each counts as one IHD.
A triple bond counts as two IHD.
count halogens as H, ignore O, count N as 1/2 H

Hydrocarbons (CxHy): IHD = (2x + 2 - y)/2 (where x and y stand for # of C and H respectively.)

index of H deficiency of saturated alkane = zero

Front 6

Electrostatic force

Back 6

force between electrons and nuclei that creates all molecular bonds

2 electrons are required to form a bond

electrons are at lowest energy level when they form a bond because they have minimized their distance from both nuclei

each bonded nuclei can donate a single electron to the bond

Front 7

Coordinate covalent bond

Back 7

one nucleus donates both electrons to the bond

Front 8

Sigma-bond

Back 8

forms when bonding pair of electrons are localized directly between 2 bonding atoms

lowest energy, most stable form of covalent bond, strong

always 1st type of covalent bond to formed between any 2 atoms

single bond must be a sigma-bond

any double or triple bond, contains 1 sigma-bond

Front 9

Pi-bonds

Back 9

additional bonds that form between 2 sigma-bonded atoms

orbital of 1st Pi-bond forms above and below sigma-bonding electrons because sigma-bond leaves no room for other electron orbitals directly between atoms

1 pi-bond = double bond (orbital above and below sigma-bond)
2 pi-bonds = triple bond (orbital on either side of sigma-bond)

weaker and more reactive than sigma-bond, but strengthen and shorten overall bond

C, N, O & S form pi-bonds

pi-bonds prevent rotation

Front 10

Bond energy

Back 10

energy necessary to break a bond

If a bond is weaker, it has a lower bond energy, and it takes less energy to break the bond

Front 11

Atomic orbitals

Back 11

s, p, d and f orbitals

The orbitals have subshells (s has 1, p has 3..)

Front 12

hybrid orbital types:

Back 12

1. sp
2. sp^2
3. sp^3

Front 13

Character

Back 13

superscripts indicate the character as follows:
sp^2 = 1s + 2p = 33% s + 66% p

hybrid orbitals resemble in shape and energy the s and p orbitals from which it is formed to the same extent that s or p orbitals are used

the more s character a bond has, the more stable, stronger, shorter the bond becomes

Front 14

hybridization, bond angles and shape

Back 14

sp = 180 = linear

sp^2 = 120 = trigonal planar

sp^3 = 109.5 = tetrahedral, pyramidal or bent

sp^3d = 90, 120 = trigonal-bypyramidal, seesaw, T-shaped or linear

sp^3d^2 = 90 = octahedral, square pyramidal or square planar

Front 15

Resonance structure

Back 15

2 or more lewis structures representing molecules with delocalized electrons

weighed average of these structures represents the real molecule (lower energy than lewis structures)

Front 16

4 rules for resonance structures

Back 16

1. atoms must not be moved (move electrons, not atoms)
2. # of unpaired electrons must remain constant
3. resonance atoms must lie in same plane
4. only proper lewis structures allowed

Front 17

2 conditions exist for resonance structures to occur

Back 17

1. a species must contain an atom either with a p orbital or an unshared pair of electrons
2. that atom must be single bonded to an atom that possesses a double or triple bond (Conjugated unsaturated systems)

Front 18

Criteria for an aromatic compound (Huckel's rule)

Back 18

The molecule is cyclic (a ring of atoms)
The molecule is planar (all atoms in the molecule lie in the same plane)
The molecule is fully conjugated (p orbitals at every atom in the ring)
4n + 2 = # of pi-electrons and n should equal any integer or 0.

Front 19

Polar molecule or bond

Back 19

molecule or bond with dipole moment

results from differences in electronegativity of its atoms

molecules with polar bond may or may not have a dipole moment

Front 20

Induced dipoles

Back 20

weaker than permanent dipoles

dipole moment is momentarily induced in an otherwise nonpolar molecule or bond by a polar molecule, ion or electric field

Front 21

Instantaneous dipole moment

Back 21

electrons in bond move about orbital, and at any given moment may not be evenly distributed between 2 bonding atoms

very short lived and weaker than induced dipoles

can act to induce dipole in neighboring atom

Front 22

Intermolecular attractions

Back 22

attractions between separate molecules

occur solely due to dipole moments

must be weaker than covalent forces (1% as strong)

attraction between molecules in proportional to their dipole moments

Front 23

Hydrogen bond

Back 23

intermolecular bond formed when H is attached to a highly electronegative atom (F, O, or N) it creates a large dipole moment leaving H with strong partial positive charge. When H approaches N, O or F on another atom, intermolecular bond is formed

Front 24

London Dispersion Forces

Back 24

weakest dipole-dipole force

between 2 instantaneous dipoles

very weak, but are responsible for phase changes of nonpolar molecules

Front 25

Isomers

Back 25

2 molecules are isomers if they have same molecular formula but are different compounds

Front 26

Conformational isomers (conformers)

Back 26

not true isomers

different spatial orientations of the same molecule

simplest way to distinguish between conformers is with newman projections (eclipsed, staggered)

Front 27

Structural isomer/constitutional isomer

Back 27

simplest form of isomer

same molecular formula but different bond-to-bond connectivity

ex:
isobutane and n-butane, both are C4H10, but have different structures

Front 28

Stereoisomers

Back 28

2 molecules with same molecular formula and same bond-to-bond connectivity that are not same compound

unless geometric isomers, stereoisomers much each contain at least 1 chiral center in same location

2 types:
1. enantiomers
2. diastereomers

Front 29

Chirality

Back 29

handedness of a molecule

chiral molecules differ from their reflections

achiral molecules are exactly the same as their reflections

Chiral compounds have absolute configs: R or S

Front 30

Absolute configuration

Back 30

physical description of orientation of atoms about a chiral center (such as a chiral carbon)

Determined by R (right) & S (left):
1. atoms attached to chiral center or #ed from higher (higher atomic weight) to lowest priority (smaller atomic weight)
2. substituents on double and triple bonds are counted 2 or 3 times, respectively
3. lowest priority group faces away
4. circle is drawn from lowest to higher priority
5. clockwise (R) and counter-clockwise (S)
6. mirror image always has opposite absolute configuration

Front 31

Relative configuration

Back 31

not related to absolute configuration

2 molecules have the same relative configuration about a C if they differ by only 1 substituent and other substituents are oriented identically about C

Front 32

Observed rotation

Back 32

direction and degree that electric field in plane-polarized light rotates when it passes through a compound. Measured using a polarimeter.

Clockwise rotation: + or D
Counterclockwise rotation: - or L

Front 33

Polarimeter

Back 33

screens out photons with all but one orientation of electric field

resulting light consists of photons with their electric fields oriented in same direction

Front 34

Plane-polarized light

Back 34

white light that has passed through a polarimeter and now has photons all oriented in same direction

Front 35

Optically inactive

Back 35

may be compounds with no chiral centers or contain equal amounts of both stereoisomers

compound does not rotate light

no single molecular orientation is favored, so there is no rotation of plane of electric field.

Front 36

Racemic mixture

Back 36

optically inactive compound

contains equal amounts of both stereoisomer therefore no rotation of light is observed

Front 37

Optically active

Back 37

compound that rotates light, orientation of electric field is rotated

racemic mixture is separated, resulting in compound containing molecules with no mirror images

if rotates plane-polarized light clockwise = + or d (right)

if rotates plane-polarized light counter-clockwise = - or l (left)

Front 38

Specific rotation

Back 38

a standardized form of observed rotation

calculated from observed rotation and experimental parameters

Front 39

Enantiomers

Back 39

same molecular formula, same bond-to-bond connectivity, not same molecule, mirror images of each other

opposite absolute configurations (R and S) at each chiral C

C-C C-C Enantiomers
R S S R

C-C C-C Enantiomers
R R S S

rotate plane-polarized light in opposite directions to an equal degree

Enantiomers have same physical and chemical properties except:
1. reactions with other chiral compounds
2. reactions with polarized light

make racemic mixture, when mixed together in equal concentrations

Front 40

Resolution

Back 40

separation of enantiomers

Front 41

Diastereomers

Back 41

same molecular formula, same bond-to-bond connectivity, not same molecule, NOT mirror images of each other

C-C C-C Diastereomers
R S R R

C-C C-C Diastereomers
R S S S

Front 42

Geometric isomer

Back 42

type of diastereomer

A type of diastereomer that exists due to hindered rotation about a bond (ring structure, double or triple bond) (cis or trans)

different physical properties

1. 2 substituents on each C are prioritized using atomic weight
2. higher priority substituent for each C on opposite sides = E
3. higher priority substituent for each C same side = Z

Front 43

Cis-isomers

Back 43

diastereomers. geometric isomers. molecules with same side substituents

have dipole moment

stronger intermolecular forces leading to higher boiling points (due to dipole moment)

do not form crystals as readily leading to lower melting points (due to lower symmetry)

steric hindrance (substituents crowd each other) of cis isomers produce higher energy levels resulting in higher heats of combustion

Front 44

Trans-isomers

Back 44

diastereomers. geometric isomers. molecules with opposite-side substituents

do not have dipole moment

Front 45

Maximum # of optically active isomers that a compound can have?

Back 45

2^n
n: # of chiral centers

Front 46

Meso compounds

Back 46

2 chiral centers in a single molecule that offset each other creating an optically inactive molecule

plane of symmetry through their centers which divides them into 2 halves that are mirror images to each other

are achiral and therefore optically inactive

Front 47

Epimers

Back 47

Diastereomers that differ at only 1 chiral C

when ring closure occurs at epimeric C, 2 possible diastereomers may be formed

Front 48

Anomers

Back 48

distinguished by orientation of substituents (in glucose, if the hydroxyl on the anomeric carbon is axial it is alpha, if equitorial it is beta)

Front 49

Anomeric carbon

Back 49

the chiral C of an anomer

Front 50

When are carbon compounds gaseous?

Back 50

Any carbon compound with less than 4 carbons is probably a gas. Greater than that is liquid/solid