Molecular Structure Flashcards
valence of: carbon hydrogen nitrogen oxygen phosphorous sulfur halogens
carbon =4 hydrogen =1 nitrogen =3 oxygen = 2 phosphorus = 3 sulfur =2 halogens are monovalent but capable of making more than 1 bond, other than fluorine)
formal charge
of e-s in isolated atom, minus the # of e-s assigned to the atom in the Lewis structure
dash formula
shows bonds between atoms but not 3D structure
condensed formula
does not show bonds, CH3NH2
bond-line formula
line intersections, corners, and ending represent C
Fischer projection
vertical lines are assumed to be oriented into the page, horizontal lines out of page
Newman projection
view straight down axis of one of the sigma bonds
index of deficiency
number of pairs of hydrogens a compound requires in order to become a saturated alkane (2n+2 hydrogens)
count halogens as hydrogens
ignore oxygen atoms
count nitrogen atoms as one half a hydrogen atom
1 degree of unsaturation
a double bond or cyclic structure
triple bond is ___ degrees of unsaturation
2
ether
R-O-R’
amine
R-N-H (-H) or with R’ and R’’
aldehyde
R-C=O(-H)
ketone
R-C=O(-R’)
carboxylic acid
R-C=O(-OH)
ester
R-C=O(-O-R’)
amide
R-C=O(-NH2)
prefix for 1-10 for alkanes
meth, eth, prop, but, pent, hex, hept, oct, non, dec
coordinate covalent bond
one nucleus can donate both electrons
electrons are at their lowest energy level when ……
when they form a bond because they have minimized their distance from both nuclei
sigma bond
bonding pair of electrons are localized directly between the two bonding atoms
most stable form of covalent bond
first type of covalent bond formed between any 2 atoms
pi bond
if additional bonds from between 2 sigma bonded atoms
each additional bond between atoms ____ the distance between the bonded atoms
shortens –> greater bond strength
___ bond are more reactive
pi bonds, electrons further from nucleus than in sigma bond, at higher energy level/less stable
character
hybrid orbital resembles in shape and energy the s and p orbitals from which it is formed to the same extent that s and p orbitals or used to from it.
sp2 = 33.3% s character and 66.7% p character
sp bond angles and shape
180
linear
sp2 bond angles and shape
120
trigonal planar
sp3 bond angles and shape
109.5
tetrahedral, pyramidal, or bent
sp3d bond angles and shape
90, 120
trigonal-bipyramidal, seesaw, t-shaped, or linear
sp3d2 bond angles and shape
90, 90
octahedral, square pyramidal, or square planar
delocalized electrons result from ….
shifting of pi bonds
resonance structures
represent molecules with delocalized electrons
4 rules for writing resonance structures
atoms must not be moved (only e-s)
number of unpaired e-s remains constant
resonance atoms must lie in same plane
proper Lewis structures used
2 conditions for resonance to occur
species must contain an atom either with a p orbital or an unshared pair of electrons
atoms must be single bonded to an atom that possesses a double or triple bond
dipole moment occurs when…
the center of positive charge on a molecule or bond does not coincide with the center of negative charge
induced dipoles
dipole moment momentarily induced in an otherwise nonpolar molecule or bond by a polar molecule, ion, or electric field
common in nature, weaker than permanent dipoles
instantaneous dipole moment
because the electrons in a bond move bout the orbital and at any given moment may not be distributed exactly between the two bonding atoms even when teh atoms are identical
can induce dipole in neighboring atom
intermolecular attractions occur…
due to dipole moments
hydrogen bond
H - F, O, or N
strongest dipole-dipole interaction
responsible for high boiling point of water
London Dispersion Forces
weakest dipole-dipole force, between 2 instantaneous dipoles
responsible for phase changes of nonpolar molecules
isomers
unique molecules with the same molecular formula, same molecular formula but different compounds
conformational isomers / conformers
different spatial orientations of the same molecule, not true isomers
chiral molecules
differ from their reflections, have “handedness”
any carbon is chiral when….
it is bonded to four different substituents
absolute configuration
only way to physically describe the orientation of atoms about a chiral center
R (clockwise circle) or S (counterclockwise circle)
relative configuration
two molecules have the same relative configuration about a C is they differ by only one substituent and the other substituents are oriented identically about the C
not related to absolute configuration
inverted in SN2 reactions
observed roation
direction and degree to which a compound rotates plane-polarized light
polarimeter
screen out photons with all but one orientation of electric field
resulting light consists of photons with their electric fields oriented in the same direction: plane polarized light
plane polarized light
consists of photons with their electric fields oriented in the same direction
optically inactive
no single molecular orientation is favored so the net result is no rotation of the plan of the electric field
may be compounds with no chiral centers or molecules with internal mirror planes (meso compounds)
optically active
when plan-polarized light is projected through such a chiral compound, the orientation of its electric field is rotated
+/d = clockwise
-/l = couterclockwise
specific rotation
standardized from
stereoisomers
two molecules with the same molecular formula and the same bond-to-bond connectivity that are not the same compound
must contain at least 1 chiral center
enantiomers
nonsuperimposable mirror images, opposite absolute configurations at each chiral carbon
rotate plane polarized light in opposite directions at an equal degree
have same physical and chemical properties except for interactions with other chiral compounds and polarized light
racemic mixture
when enantiomers are mixed together in equal concentrations
do not rotate plane polarized light
resolution
separation of enantiomers
diastereomers
not mirror images, not same compound
same molecular formula and same bond to bond connectivity
ex: geometric isomer
geometric isomer
due to hindered rotation around double bold
cis vs. trans
*have different physical properties
___ molecules have dipole moment while ___ molecules do not
cis molecules have dipole moment, trans do not
melting and boiling points of geometric isomers
cis have higher boiling points: dipole moments –> stronger intermolecular forces
cis have lower melting points: lower symmetry –> do not form crystals as readily
E vs Z
E = highest priority (greatest weight) on opposite sides Z = on zame side
max number of optically active isomers =
2^n (n= number of chiral centers)
meso compounds
two chiral centers in single molecule offset each other creating an optically inactive molecule
have plane of symmetry through center that divides them into two halves that are mirror images of each other
*achiral and optically inactive
epimers
diastereomers that differ at only one chiral carbon
anomers
two possible diasteriomers that can form if a ring closure occurs at the epimeric carbon
anomeric carbon = chircal carbon of anomer
