OChem Exam 1 Flashcards
Isomers
compounds that are constructed from the same atoms (and molecular formula) but differ from each other
1. constitutional
2. stereoisomers
constitutional isomers
same formula, differ in order of bonding
stereoisomers
same formula, different in 3D arrangement
cis
same side of double bond
trans
opposite side of double bond
chiral
a molecule that is not superimposable (able to be laid upon) of its mirror image
achiral
a molecule is superimposable on its mirror image
enantiomer
a non-superimposable mirror image of a compound (same physical properties)
diastereomer
non-superimposable and NOT mirror images (has different physical properties; cis-trans isomers)
meso compound
has reflectional symmetry and is achiral even with chiral centers (has a plane of symmetry)
chiral molecule
has the ability to rotate plane-polarized light
chiral center
tetrahedral atom attached to 4 different substituents
hyperconjugation
donation of one electron density from an adjacent sigma bond to stabilize an electron-deficient pie system
Name the steps to drawing a lewis structure
- count total number of valence electrons (based on group number)
- pick central atom (most EN)
- build sigma bidn framework
- fill outer octets (most EN first)
- remaining lone pairs on central atom
- make pi bonds if central atoms does not have octet
- assign formal charges (circle)
How to calculate formal charges
valence electrons (PT number) - electrons “owned” (e on structure)
define electronegativity
the tendency of an atom, in a covalent bond, to draw electron density towards itself.
*most EN atom is F, as EN increases up a group and across a row
lewis structure exceptions
- odd number of electrons
- expanded octet (common on P, S, Cl, Br, I)
- unfilled octet on central atom (common on transition metals)
- oxyacids (H leading with many Os- H are attached to outer atoms, not central atom)
How to find the most polar molecule?
- The greater the electronegativity difference, the more polar the bond.
- linear and symmetrical molecules tend to be nonpolar, while asymmetrical molecules tend to be polar.
- If the individual bond dipoles within the molecule do not cancel each other out due to symmetry, the molecule will have a net dipole moment, making it polar.
Define polarizability
the ability of an electron cloud to distort in response to the approach of a point negative charge
*most polarizable is bottom left because larger molecules are more polarizable where electrons can move away in the cloud from approaching negative charge
Line structure vs condensed formula
line structure: each point is an assumed carbon
condensed formula: shows no bonds. for branches: main(branch)continue
rings or acyclic structures can be abbreviated, how?
as Ph-
*put “letter -“ in front of a formula to indicate ring formation like o, m, or p
p-
para– 1,4 arrangement
m-
meta— 1,3 arrangement
o-
ortho– 1,2 arrangement
Bronsted-Lowery Acids and Bases defintion
acid= proton donor
base= proton acceptor
(H+ is a proton)
Lewis acid and base definition
acid: electron pair acceptor (electrophile)
base: electron pair donor (nucleophile)
Luse Flood acid and base definition
acid: oxide acceptor
base: oxide donor
What are the 4 factors that influence acid strength? (in the order of importance)
- resonance
- polarizability
- electronegativity
- s-character
explain how resonance impacts acid strength
more oxyegn= more resonance= stronger acid due to a stable anion
explain how polarizability impacts acid strength
bigger atom= lower-left corner of PT= stronger acid
explain how electronegativity impacts acid strength
more EN= upper right PT= more positive nucleus= electrons held tighter= stronger acid
explain how s-character impacts acid strength
s-character= orbital size= small orbitals hold electrons tighter= stronger acid
How to draw 6 Newman projections for a molecule
- choose a bond to look down and orient
- Draw Front Carbon: dot
- Draw Rear Carbon: Larger circle
- Position Substituents
- rotate the front carbon by 60 degrees each time
- ID most and least stable
- rank in stability and put on an energy diagram
define Torsional Strain
close electron clouds, like in eclipsing conformation, cause repelling
*gauche torsional strain: non H compounds are next to each other and not eclipsed
define steric strain
intersection of electron clouds causes strain
define transannular strain
when 2 groups point at each other from across a ring with an even number of carbons (chair conformation)
how to number a carbon ring
based on the most highly oxidized carbon (more O) or the higher priority substituent
how to find the most stable chair
- sp3 hybridized atoms (alkyl groups - CH3) have the highest strain
- sp2 hybridized (acids, ester)
- nitrogen and oxygen substituents
- halogens
*most energy and biggest substituent has associated with 1,3 diaxial strain– preference for equatorial position
name the 1-10 carbon prefixes for naming
1– meth
2– eth
3– prop
4– but
5– pent
6– hex
7– hept
8– oct
9– non
10- dec
Steps of Free Radical Chain Mechanism
- Initiation: formation of free radicals by breaking a covalent bond, often through heat or light. Cl₂ → 2 Cl*
- Propagation: repeated step involves the successive generation and consumption of free radicals– creating new radicals in the process. CH₄ + Cl* → CH₃Cl + H*
- Termination: involves the combination of two free radicals to form a stable molecule, thereby removing radicals from the chain. 2 Cl* → Cl₂
- Overall: CH₄ + Cl₂ → CH₃Cl + HCl and H△ rxn overall
*don’t forget to show fish hook arrows for electron movement and hv above the rxn arrow
Define the Hammond postulate
“If two states, as for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of molecular structure”
TLDR: close in energy= close in structure
steps for naming an enantiomer
- find chiaral centers
- number based on priority (atomic number)
- rearrange so 1 is coming out and 4 is going back
- connect 1 to 2 to 3 in 3D
- clockwise = R and counterclockwise = S
calculating enantiomeric excess %
Enantiomeric excess (ee): the amount of pure isomer after subtrating out the racemic part
ee (%) = ([specific rotation]measured / [specififc rotation]pure) x 100%
- higher ee% indicates a greater purity of the desired enantiomer
Steps to pick the best chair conformation and calculate the approximate ratio of the two
- draw as a ring and make chairs
- ID strain and use given ΔG to find most and least stable
- find ΔGrxn = more - less
- use answer in Keq equation
- solve Keq to get answer to plug into Keq = Product/Reactant
- plug that answer into product + reactant = 1.0
- convert decimal to percent
Gibbs free energy (ΔG):
R is the ideal gas constant– 8.314 J/(mol·K)
T is the temperature in Kelvin (298 K).
Keq = e^(-ΔG° / (RT)) *delta G is NEGATIVE
Keq = [products] / [reactants]
[products] + [reactants] = 1.0
make a resolution (separation of a racemic mixture) flow diagram
- racemic mixture (enantiomers) with the same physical properties
- use optically pure reagent to separate mixture into diastereomers with different physical properties
- amine (N-H) group’s electron pair interacts with the reagent - separate diastereomers and cleave them into pure isomers and reagents
- recover reagents for reuse or throw away
define racemic mixture
a 1:1 mixture of enantiomers that rotates plane-polarized light
define what a C2 axis of symmetry is
if you perform a half-turn rotation (180 degrees) around the C2 axis, the object will superimpose onto its original orientation
name the 3 ways to separate or do the resolution of enantiomers
- crystalization (physically separate)
- chiral resolving agents (use a reagent and separate diastereomers)
- column chromatography (separate by interaction (may stick) with a column of medium)
define conformational isomer
a stereoisomer that can be interconverted by rotating about sigma bonds (no bond breaks)
what types of strain cause one conformation isomer to be higher in energy than another?
- eclipsing torsional strain
- gauche torsional strain (60 degree dihedral angle)
- eclipsing steric strain
recreate relative reactivity chart
F2 Cl2 Br2
1- 1.0 1 1
2- 1.2 4 80
3- 1.4 5 1700
how to find a specific rotation
[a]specific = ([a]observed x100) / ( ℓ x c)
ℓ= length of polarimeter in dm
c= concentration in grams/dL
