ochem 1 Flashcards
What is stronger a sigma bond or a pi bond? Why?
sigma is stronger. allow for electron density to be concentrated to a much larger degree between 2 nuclei. pi bond weaker because the bond is in a higher energy state
the first bond formed between two elements is a __ bond
sigma
As the radius of either atom increase, the p orbitals are ___ ___, resulting in less __. leading to a ___ pi bond
spread apart, overlap, weaker.
are sigma bonds or pi bonds more reactive?
pi bonds
What is hybridization?
Atoms, when bonded, hybridize (i.e., mix) their higher and lower energy valence electron orbitals to form “hybrid orbitals” with intermediate energy
How to determine hybridization
count number of electron densities and minus 1
What is the percent of s character of the hybrid oxygen orbital in water?
sp3, 25% s character
What is the Valence Shell Electron Pair Repulsion Theory?
The theory that predicts which shape molecules will take
due to the repulsion of lone pairs of electrons
name the shapes and bond angles of sp, sp2, sp3, sp3d, sp3d2 orbitals
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 ̊or180 ̊]
sp3d2= Octahedral, Square Pyramidal, or SquarePlanar [90 ̊]
order the types of bonds (single, double, triple) in decreasing bond length
single, double, triple
Rank the bonds (single, double, triple) according to increasing stability
single, double, triple
Rank the bonds (single, double, triple) according to increasing reactivity.
single, triple, double
Rank the bonds (single, double, triple) according to increasing bond strength.
single, double, triple
Describe the forces responsible for the strength of a bond. What is the relationship between potential energy and bond length?
A chemical bond is a balance of electrostatic forces. We know that species with opposite charges (i.e., nuclei and electrons) will be attracted to each other with a force given by Coulomb’s Law: F = K(qq/r2). However, remember that there are also repulsive forces between the two nuclei and between electrons. The bond is formed at a sort of “sweet spot” where potential energy is lowest. Increasing bond length or decreasing it would raise the energy of the bond and therefore weaken it.
Which atom is the most electronegative?
fluorine
Which atom is the least electronegative?
francium
How to calculate dipole moment?
u= sigma(d)
sigma = charge d = distance between charges
molecules with __ or more dipole moments can have NO __ __ when their geometric orientation causes the dipole moments to __ each other out
2, net dipole, cancel
energy is always __ when a bond is formed, and ___ when a bond is broken
released, required
The less stable the bond, the ___ the heat of combustion
greater
the more stable the bond, the ___ the heat of combustion
lower
What is a coordinate covalent bond?
bond which both electrons shared in the bond are donated by one atom.
What is the octet rule?
atoms of low atomic number (<20) tend to gain or lose electrons to obtain exactly 8 electrons in their valence shell
exceptions to octet rule
1) Hydrogen and Helium: Stable with only two electrons in their valence shells (e.g., H2)
2) Boron and Beryllium: Stable with only six electrons in their valence shells (e.g., BF3)
3) Atoms from the third period or higher can accept more than eight electrons. Common MCAT examples include: PCl5, SF6, PO43- and SO42-.
What is formal charge?
The difference between the # of electrons in an atom’s valence shell when it is in its ground state and the number assigned to it in a molecule.
equation for formal charge
formal charge = valence - assigned
regarding resonance, the actual structure ___ resonate back and forth between forms, it is a ___ weighted hybrid of the contributing structures
does NOT, permanent
Drawing resonance structures rules
Atoms can never be moved.
Single bonds can never be moved.
All structures must obey the octet rule
All structures must have the same number of total valence electrons
The tail of an arrow showing electron flow during resonance can only start from a lone pair, a double, or a triple bond
ranking resonance structures rules
- allows the most atoms to have a full octet
- has least formal charge
- places formal charge on most electronegative atom
more stable species, has more __-
resonance
What is Huckel’s Rule?
To exhibit aromaticity, a ring system must have exactly 4n + 2 pi electrons
What does alkoxy look like?
- O–R
acetyl group
Ch3-C=O
What are isomers?
Two molecules are isomers if they have the same molecular formula but are actually different
compounds.
How to calculate the maximum number of optically active stereoisomers
2^n
n= chiral centers
What is a structural isomer
Same formula, different bond-to-bond connectivity
what are Stereoisomers
Same formula, same bond-to-bond connectivity, but different in the 3-D arrangement of their substituents. There are two categories of stereoisomers: enantiomers and diastereomers
what is Relative Configuration
Two molecules have the same relative configuration if their spatial arrangement is identical, but they have one—and only one—non-identical substituent
what is observed rotation
the degree to which a sample rotates plane polarized light
What is specific rotation?
dividing observed rotation by the length of the tube and the concentration. Specific rotation could therefore be described as “observed rotation per length, per concentration unit.”
What is a polarimeter?
apparatus that measures the rotation of plane-polarized light as it passes through a sample
What is plane polarized light?
light that exists in only a single plane
What does optically active and inactive mean?
active - substance does rotate plane-polarized light
inactive- doesn’t
what is a racemic mixture?
50/50 mix of the two absolute configurations of a compound (R and S)
If a compound rotates light clockwise it is ___
+ or d (dextrorotary)
If a compound rotates light counter clockwise ____
(-) or l (levorotary)
What are diastereomers?
Two molecules with the same formula and the same bond-to-bond connectivity that are non-identical, but are NOT mirror images.
What are geometric isomers?
(cis/trans): cis = same side; trans = opposite sides.
The E/Z Convention: prioritize the two constituents on each carbon by molecular weight (as you do for R/S). 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 often have a __ __, but trans do NOT
dipole moment
What are epimers and anomers?
epimers- Diastereomers that differ at only one chiral center. Many pairs of carboydrates are epimers (e.g., glucose and galactose).
anomers - Molecules that differ only in their spatial orientation at the anomeric carbon of a ring structure. If the anomeric OH/OR group and the CH2OH group are on the same side of the ring = Beta, if they are on the opposite side = Alpha.
What are meso compounds?
molecules w 2 or more chiral centers that contain a plane of symmetry. This symmetry cancels out their optical activity
nucleophiles always attack ___
electrophiles
a strong base favors the ___ in acid/base equilibrium
conjugate acid
nucleophilicity is a function of ___
kinetics - how easily a molecule will react , nothing about how stable the new bond will be
electrophiles are ___ species with a full or partial __ charge
electron poor, positive
What are leaving groups?
atoms or molecules that leave the parent molecule during a reaction and take both electrons from the bond w them.
what are the steps of E1
1) The dissociation of the leaving group, resulting in formation of a carbocation [slow step].
2) The abstraction of a proton with collapse of the electrons to form a double bond (which
quenches the carbocation) [fast step].
What are the steps for E2?
1) The single-step abstraction of a proton with collapse of the electrons to form a double bond and ejection of the leaving group.
What are the steps in SN1?
1) The dissociation of the leaving group, resulting in formation of a carbocation [slow step].
2) Attack of the carbocation by the nucleophile [fast step].
What are the steps for SN2?
1) The single-step “back side attack” of the electrophile with simultaneous ejection of the leaving group.
E1 is __ order and is favored by ___
first; weak bases, tertiary carbons only, polar protic solvents
SN1 is a ___ order reaction, it is favored by ___
first; poor nucleophiles, tertiary carbons, polar protic solvents
E2 is a ___ reaction, and is favored by ____
second, strong and/or bulky bases
SN2 is a ___ order reaction, it’s favored by ___
second; good nucleophiles, methyl, primary or secondary carbons
(inversion)
What are alkanes?
Compounds made entirely of carbon-carbon or carbon-hydrogen single bonds (e.g., gasoline,
tar, crude oil, butane, methane, etc.)
alkanes are ___ in water, ___ density, and have what kind of polarity?
insoluble, low, non-polar
boiling pt increases with ___ chain length
increasing
boiling pt decreases with __ branching
increased
melting point increases with ___ chain length and/or molecular weight
increasing
___ alkanes have the highest melting points. Among branched alkanes, __ branching increases melting point
straight-chain, increased
What is ring strain regarding cyclic compounds?
cycloalkanes create ring strain because they force bond angles to deviate from the optimum tetrahedral angle of 109.5°
bicyclic ring systems generally exhibit __ ring strain than do monocyclic rings
more
are large substituents more stable at the equatorial or axial position?
equatorial
rings near __ to __ members or above will have ring strain equal to that of cyclohexane
10-12
How are radicals formed?
homolytic cleavage sends one electron to one species and one to another
How to make a alkane from an alkene
CH2=CH2 + H2/Pd(catalyst) → CH3CH3
syn addition
What is more stable? A tri-substituted alkene or a di-substituted alkene?
trisubstituted
What is the difference between a phenyl and benzyl? What is the molecular formula for a benzene?
phenyl - benzene directly attached to primary chain
benzyl- a CH2 is attached to the primary chain on one side and benzene on the other
C6H5
Alcohol can behave as a __ or a ___
nucleophile, lewis acid
For alcohols, increasing molecular weight ___ boiling pt, boiling pt decreases when branching ___
increases, increases
melting pts of alcohols go up when polarity and H-bonding ___
increases
hydrogen bonding can occur with what atoms?
F, O, N
Why are most alcohols highly soluble in water?
alcohol can hydrogen bond with water
alcohol acidity increases from __ to __ to __
tertiary, secondary, primary
formation of an alkyl halide from an alcohol via SN2
CH3OH + HCl → CH3OH2+ + Cl- → CH3Cl + H2O
formation of an alkyl halide from an alcohol via SN1
Via SN1: R3COH + HCl → R3COH2+ + Cl- → R3C+ + H2O → R3CCl
What are the common oxidizing agents for alcohols
O3, Cr2O7, CrO4, KMnO4, Jones, Collins, PCC, PDC
primary alcohols can be oxidized to __ and __
aldehydes and carboxylic acids
secondary alcohols can be oxidized to __
ketones
tertiary alcohols can be oxidized to __
cannot be oxidized further
What are common reducing agents?
NaBH4, LiAlH4 and H2
NaBH4 can reduce __ and __
aldehydes and ketones
LiAlH4 and H2/pressure can reduce __
aldehydes, ketones, carboxylic acids and esters
LiAlH4 produces __ equivalents of hydride ions and NaBH4 produces __ equivalents
2, 1
What is the pinacol rearrangement?
vic-diol + hot acid → ketone or aldehyde
How to protect alcohols
Protection with TMS: ROH + TMS → RO-Si(CH3)3
• Protection by MOM (Mothers are very protective! MOM stands for methoxymethyl ether):
ROH + Base → RO- + CH3OCH2Cl (MOMCl) → ROCH2OCH3 (RO-MOM)
• Acidification will remove either protecting group to restore the alcohol.
reaction with SOCl2 and PBr3
o ROH + SOCl2 → RCl
o ROH + PBr3 →RBr
What are the steps to form a tosylate or mesylate?
1) The alcohol attacks the tosyl or mesyl halide via SN2, kicking off a halide ion.
2) A hydrogen is abstracted by the halide ion, quenching the charge on the oxygen.
Why would we want to form a tosylate or mesylate?
they are good leaving groups that readily react with almost any nucleophile
name the steps of the dehydration of an alcohol to an alkene
1) The alcohol is protonated by the acid
2) The “good leaving group water” leaves, forming a carbocation
3) Methyl or hydride shifts can occur, but only if it results in a more stable carbocation.
4) A water molecule abstracts a proton and the electrons collapse to quench the carbocation and
form an alkene.
the alkene is favored by __, __ acid ; the alcohol is favored by __, __ acid
hot, concentrated
cold, dilute
What is the purpose grignard synthesis?
Production of an alcohol with extension of the carbon chain
What are the steps of a Grignard reaction?
1) Due to the very low electronegativity of Mg, the R group in RMgBr gains significant electron density and more or less acts as if it were a carbanion (R:-), attacking the electrophilic carbonyl carbon. This occurs in a single step, kicking the electrons in the C=O bond up onto the oxygen.
2) The negatively charged oxygen is protonated, yielding an alcohol.
What is an ether?
An ether is an oxygen bonded to two –R groups (R-O-R)
What are some properties of ethers?
o Very non-reactive
o Weakly polar
o With short –R groups ethers are slightly soluble in water o Most non-polar species are soluble in ethers
o Low boiling point (no H-bonding)
ethers are excellent __
solvents
What is an epoxide?
Epoxides are cyclic ethers involving one oxygen and two carbons in a three-member ring.
How to open an epoxide
Epoxide rings can open via either an SN2 mechanism with good nucleophiles, or an “SN1-like” reaction with poor nucleophiles. In SN2 reactions the good nucleophile attacks the least substituted epoxide carbon and in SN1-like reactions the nucleophile attacks the carbocation formed on the most substituted epoxide carbon.
the greater the difference in the __ of two atoms in a bond, the more polar the bond
electronegativity
How to determine if a group is electron donating or withdrawing
1) Look at the first atom from the point of attachment. Compare its electronegativity to the atoms bound to it. If it is more electronegative, it will bear a partial negative charge and if it is less electronegative, it will bear a partial positive charge.
2) Atoms with full or partial positive charges withdraw from whatever they are attached to. Atoms with full or partial negative charges donate to whatever they are attached to.
3) Hydrogen is considered neither electron donating nor withdrawing.
4) Alkenes are weakly electron withdrawing (just memorize this one).
Label each of the following as electron donating or electron withdrawing: alkyl groups, nitro groups, cyano groups (a.k.a. nitriles), sulphones, amines, carboxylic acids, esters, alcohols and quaternary amines.
Alkyl - weakly electron donating nitro - strong withdrawing groups cyano - electron withdrawing sulphones - electron withdrawing amines - electron donating carboxylic acids - electron withdrawing esters - electron withdrawing (assuming connection to the carbonyl; if connected to the oxygen of the –OR group, an ester is electron donating); alcohols are electron donating; quaternary amines are strongly electron withdrawing.
What to look for when ranking acidity
look at stability of the conjugate base, resonance stabilization
How to rank basicity
Electron donating groups increase basicity, while electron withdrawing groups decrease basicity.
steric hindrance favors __ over ___
basicity, nucleophilicity
reactivity favors __ over __
basicity, nucleophilicity
