chem 14c – Chemistry

an ester contain within a ring
bond angles
hybrid orbitals
geometry of hybridization
torsional strain
e- repulsion when an atom passes by another atom/group by rotating
-rotation is allowed as long as it does not significantly decrease orbital overlap.-in newman projection, it is motion of getting the molecules to line up
steric strain
strain caused by e- repulsion on an atom/group by another near by atom or group.
-in newman projection, it is when the substituents are line up
angle strain
strain resulting from non ideal angles.
ring strain
strain from being in a ring, combination of torsional and angle strain.
functional groups
group of atoms that determine molecular properties ( chemical, physical, or biological)
-allows use to make predictions about a molecule.
-chemical rxn is influence by e- distribution and molecular structure.
gauche,stagger, ecilpse.
gauche-biggest groups 60 degrees next to each other
stagger- evenly spread out
-has equal bond angles
-permit p orbital overlap=inc stability
-max p orbital overlap occurrs when p orbitals are parallel(that is when they are planar)=max bond strength and max stability
-conjugations have partial pi bonds (dotted lines in resonance hybrid) that restrict rotation and force planarity
-if there is significant torsional strain, the molecule won’t be planar.
axial vx equatorial
axial up and down, equitorial=left/ right
when a molecule can be represented in 2 or more valid lewis structure.
– must have a pi orbital, except in rare case where sigma bond break for resonance.
-allows electron delocalization
-e- are delocalize and stablize by resonance
-less e- repulsion the more stable the molecule.
-chemical reaction often produce stable products ( resonance, conjugation, etc)
Resonance contributor
one lewis structure that contributes to resonance
-has no physical reality
resonance hybrid
a weighted avg of resonance contributor.
– single lewis dont strucutre that takes all of the resonance contributors into account.
-look at this when considering any property (ie: bond length) instead of individual resonance contributor.
-helpful way to locate conjugate atoms
patterns of resonance:
1. lone pair<--> pi bond
2. open octet<--> pi bond
3. pi bond <--> lone pair and open octet
4. aromatic ring pi bond circulation ( rotation of pi bond in one ring at a time)
rules of resonance
1- most important- full octet
2. max # of covalent bond
3. least number of formal charge.
4. negative charge are best on the most electronegative atom and positive charge are better on the least electronegative ( -)
5. resonance is strong between elements in the same row.
barrier to rotation:47
-partial pi bond=higher barrier to rotation=planarity=stability
heat of formation(ΔHf)
enthalpy change when a substance is made from an element in their standard state. the smaller the ΔHf means the more stable a molecule is.
catalytic hydrogenation
-the adding of a hydrogen, disrupts conjugation ,but doesn’t completely destroy it.
-the lower the entalpy of hydrogenation the more stable the isomer is.
-the special stability provided by 3 or more adjacent, parallel, overlapping p orbitals
-has partial pi bonds that restrict rotation and force planarity( result of conjugation)
-increase stability
-a planar conformation is required to achieve conjugation
-require atoms to contribute a pi bond (sp and sp2 hybridization)
-unlike resonance, conformation isomers are real structures with distinct lifetimes.
colors in conjugation
more conjugated= more color(visible light)
ΔE= HOMO/LUMO gap ( change in energy from ground state to excited state)
-high HOMO/LUMO= uv light
inc pi bond=dec. HOMO/LUMO gap=dec. in energy required to excite an electron
-there is no relationship between the stability of a molecule and its color. stability is determined by the energy of the molecule and color is determine by the difference of energy between 2 molecular orbitals
partial pi bonds(dotted lines in resonance hybrid)
-shorter than single bonds, longer than a carbon-carbon double bond
-partial pi bond=higher barrier to rotation=planarity=stability
-two adjacent sp2 atoms have a partial pi bond(or full) due to p orbital overlap
-rotation of sigma bond and torsional strain decrease p orbital overlap destroying partial pi bond and dec. the stability
pg: 65
when torsional strain destabilization < conjugation stabilization=coplanar(dihedral angle of 0 degrees
-is perpendicular if torsional strain destabilization> conjugation stabilization( ie. 2 methyl group repulsion)
the extra stability possessed by a molecule that meets the following criteria:
1. cyclic/ ring structure
2.must have a p orbital
3. pz orbital must overlap and be planar(or close to it)
4. orbital arrangement must result in a lowering of energy: 4n+2
– prevented by torsional strain or steric strain b/c they interfer with p orbtial overlap
-ring strain usually doesn’t interfere with p orbital overlap… so doesn’t prevent aromaticity.
diradical pg: 69
a species with two unpaired electron
the presence of pi bonds is a result of hybridization not a pi bonds cause hybridization.
absolute configorations
relative spatial postion of attachments at the sterocenter. assign r for clockwise and s for counter clockwise.
-non superimposable mirror image
-superimposible mirror image = achiral
-allows optical activity
priority rule
lowest priority and in the back for atoms with the lowest atomic number.
lone pairs have no protons or nucleus so its priorit is always lowest.
stereoisomer that is not an enantiomer
one of a pair of stereoisomer that are non superimposable mirror images
– only physical property of enantiomer is the direction the rotate plane-polarized light
meso compound
achiral compound that contains two or more sterocenter
optically active
-rotates plane-polarized light
carbon atom with four different attachments
-2^n stereocenters at most?
-same molecular formula,but different orientations
a. conformational b.geometrical isomer(14b)
c. optical isomers-two types: diastereomers and enantiomer)
one of a set of isomer of identical constitution(sequence of atomic attachments) but differing in the arrangement of their atoms and non interconvertible by rotation around a single bond
biological effect
-when a compound interacts with an enzyme or receptor in the body
-enzyme and receptors are constructed by proteins and have very specific shape and size
constitutional isomer
same molecular formula, but different connectivity
conformational isomers
different rotation around a single bond
-has a physical reality, unlike resonance
anomeric carbon
fisher projection
haworth projection
α and β and D configuration
-refers to the stereochemistry at the anomeric carbon
d= means the carbohydrate has the most common natural configuration
-alpha is easier to break down than beta
α if when the CH2OH and OR groups are trans in the α anomer.
-β when CH2oh and anomeric group are cis in the β anomer

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