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Dangerous Conversationalist
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 Posted: Mon Jul 15, 2013 8:58 pm
Arrhenius - acid = increase H+ in solution (arrhenius cares about ions in aqueous soln)
base = increase OH- in solution
Bronsted-Lowry - acid = proton donor (bronstead cares about protons)
base = proton acceptor
Lewis - acid = e- pair acceptor (lewis cared about e-)
base = e- pair donor
In strong acids, the reactant will always react fully to create H+
In strong bases, the reactant will always react fully to create OH-
Na2O will equal TWICE the base concentration: [OH-] = 2* [Na2O]
i.e. 0.1 M Na2O, pH =?
-log[pOH] = -log[2 x 0.1] = - (log 0.2) = -(log 2 - log 10) = 0.7
pH = 14 - 0.7 = 13.3
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Posted: Mon Jul 15, 2013 11:16 pm
Conjugate Base / Acid
Conjugate base - s**t produced when acid LOSES a proton (- H+)
Conjugate acid - s**t produced when base GAINS a proton (+ H+)
i.e. HF --> F- Conjugate acid = HF, conjugate base = F-
The conjugate base of a strong acid is neutral.
The conjugate acid of a strong base is neutral, since reverse reaction will not occur (reaction will go forward completely in strong acid/base)
For weak acid/bases, the conjugate base of a weak acid is a STRONGER base than water.
The weaker hte acid, the stronger the conjugate base (than water).
The conjugate acid of a weak base is STRONGER acid than water. The weaker hte base, the stronger the conjugate acid (than water).
THE STRONGER THE SPECIES, THE WEAKER THE CONJUGATE; THE WEAKER HTE SPECIES, THE STRONGER THE CONJUGATE
Recap: Conjugate base of a strong acid is pH neutral - nothing happens in water
Conjugate acid of a strong base is pH neutra - nothing happens in water
Conjugate base of a weak acid is a weak base - behaves as base in water
Conjugate acid of a weak base is a weak acid - behaves as an acid in water
i.e. Does NH4Cl make acidic, basic, or pH neutral solution when dissolved in pure water?
Cl- is the conjugate base of a strong acid; no effect on pH
NH4 is conjugate acid of a weak base; will have acidic properties in water; soln will be acidic
Ka (acid) x Kb (its conjugate base) = Kw = 1 x 10 -14
pKa + pKb = 14
BUFFER SOLNS - one that can resist large changes in pH when small amounts of acid or base are added.
Buffer soln consists of a WEAK acid and its conjugate base // or WEAK base and its conjugate acid in approx equal concentrations.
Acid's job is to neutralize any added base, and hte base's job is to neutralize any added acid
H3O+ + OH- --> 2 H2O
buffer provides cushion to pH; soln of a weak acid in equilibrium with its conjugate.
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Dangerous Conversationalist
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Dangerous Conversationalist
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Posted: Mon Jul 15, 2013 11:28 pm
Neutralization - when an acid and base combine with water, you get water + conjugates ofe the original acid and base
Salt is composed of an anion and cation species resulting from acid-base neutralization rxn!
a x MaVa = b x MbVb
a = # hydronium ion can make per mol
b = # of OH- ion can make
THERMODYNAMICS
1 cal = 4.184 J
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Posted: Tue Jul 16, 2013 12:34 am
First law of thermodynamics: nrg is neither created nor destroyed
Second law of thermodynamics: nature acts to minimize nrg and maximize entropy
Entropy is favored when S>0
Enthalpy favored when H<0
All processes that are both entropy favored and enthalpy favored are SPONTANEOUS
Gibbs free nrg
G = H - TS
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Dangerous Conversationalist
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Dangerous Conversationalist
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Posted: Tue Jul 16, 2013 2:51 am
Galvinic Cells
anode | ionic species in soln in anodic half-cell || ionic species in soln in cathodic half-cell | cathode
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Posted: Tue Jul 16, 2013 3:17 am
Finally... the awaited ochem... ;_; shoot me now
Saturation
2n+2 H atoms, n = carbons
#H < n carbons means unsaturated
One degree of unsaturation = one pi bond or one ring
2 degree unsaturation = 2 pi bond (2 seperate db or 1 tb), or 1 pi bond and 1 ring, or 2 rings
etc
Degree of unsaturation = (2n+2) - x / 2
x: # hydrogens and any halogen (Cl, Br, I)
n: # carbons
Ignore oxygen
Replace N with 1C and 1H
degree of unsaturation of 0 = saturated
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Dangerous Conversationalist
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Dangerous Conversationalist
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Posted: Tue Jul 16, 2013 11:50 pm
Structure & Bonding
Resonance structures have 3 criteria:
1) resonance contributors inw hich octet rule is satisfied
2)Resonance contributors that minimize separation of charge are better than those with a large separation of charge
3) In structures that have separation of charge, the more important resonance contributor has negative charge on the more electronegative atom, and positive charge on the less electronegative ion
Usually involves e- that are adjacent (one atom away from) a pi bond or unhybridized p orbital
Resonance structures can NEVER be drawn through atoms tha tare truly sp3 hybridized.
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Posted: Wed Jul 17, 2013 1:16 am
In order to find the stronger acid, you must find the conjugate base of each acid to see which will have the more stabilized anion. Once you realize that both conjugate bases have e- that can be delocalized, question is: which conjugate base is more stabilized by resonance delocalization?
i.e. Phenoxide Ion vs Acetate ion
Whichever has the stablest anion = stronger.
Since resonance for phenoxide ion disrupts the benzene's aromatic structures, and also has a negative charge on carbon rather than the electronegative oxygen, acetate ion is the stronger acid. Both resonance structures on the acetate ion are of equivalent energy.
remember HBr is a strong acid
Also remember:
**Bronsted-Lowry says an acid is a molecule that donates H+ and takes on a negative charge. The extent to which a negative charge is stabilized determines how strong the acid is.
Therefore, make sure when figuring out relative strengths of acids, if you see a molecule THAT DONATES H+ ADJACENT TO SP3 ORBITAL, REMEMBER THAT IT CANNOT MAKE RESONANCE STRUCTURES!!! <--- therefore will ALWAYS have the weakest acidity
REMEMBER:
Acidity:
H2So4, HCl, HBr, HI, HNO3 > sulfonic acids > carboxylic acids > phenols > alcohols/waters > sp C-H bonds > sp2 C-H bonds > sp3 C-H bonds (LAST)
Bond length
The greater the s character in component orbitals, the shorter hte bond. triple bond has a tighter and shorter bond.
i.e. C-C bond (sp - sp) has 50-50 s orbital, will be tighter than
C-C bond (sp - sp3), since sp3 has 1/4 s orbital and 3/4 p orbital
Isomerism and Chirality
Structural isomers - compounds that have the same molecular formula but whos atoms have different connectivity
Conformational isomers - compounds that have the same molecular formula and hte same atomic connectivity but differ by rotation about a sigma bond (single bond).
i.e. C2H6
This can be rotated to form 2 different conformations
staggered conformation vs eclipsed conformation
Sterioisomerism - molecules that have same molecular formula and connectivity but differ from one another in spatial arrangement of atoms. They CANNOT be interconverted by rotation of single bonds like conformational isomerism
Chirality - any molecule that can't be superimposed on its mirror image is chiral; have 4 diff groups bonded to it
Achiral - molecule that can be superimposed on its mirror image and has a plane of symmetry (you can cut molecule in half and get same)
Possible stereoisomers = 2^n, where n = # chiral centers
Cahn-Ingold-Prelog rules
1. Prioritize the 4 substituents accoridng to increasing atomic number
2. move lowest priority to back
Also note: D = isotope of hydrogen, has an extra neutron. Tritium is hydrogen with 2 neutrons. lowest priority will be H> D> T
- If 2 identical atoms are attached, next atoms in both chains are examined until a difference is found
- A multiple bond is counted as 2 single bonds (will be bootleg version) so real version is awlays higher priority than the bootleg
3. See if it is R - right or S (left) - sinister
Enantiomers - nonsuperimposable mirror images (like your hands)
i.e. SS and RR / RR and SS
Diastereomoers - stereoisomers that are not enantiomers; non-superimposable non-mirror images
i.e. SS and RS and SS SR
Recap: S to R = enantiomer
SS to SR = diastereomer
R to R = identical
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Dangerous Conversationalist
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Dangerous Conversationalist
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Posted: Wed Jul 17, 2013 1:49 am
Compounds that rotate the plane of polarized light = optically active
Racemic mixture = a 50/50 mixture of 2 enantiomers. They are optically inactive since one enantiomer will rotate plane polarized light in one direction, while the other rotate light in the same magnitude in the opposite direction.
When 2 chiral centers have same groups attached, ther emaybe an internal plane ofsymmetry. IF SO, instead of 2^n stereoisomers possible, 2 will be identical and superimposable due to internal plane of symmetry within the molecule. They will only have 3 stereoisomers instead of 4
meso compound - compound that has an internal plane of symmetry in a molecule that cotnains chiral centrers
usually has R S against mirror image
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Posted: Wed Jul 17, 2013 2:24 am
Structure and reactivity of Alkanes and Cycloalkanes
Alkanes are usu unreactive. 2 reactions that can happen: COMBUSTION with oxygen and FREE RADICAL HALOGENATION
Structural conformations: anti (staggered but 180 deg, most sterically favorable and lowest energy)
gauche (60 deg apart)
With cyclohexanes, and other cycloalkanes, the conformation more stable is one iwth large groups n equatorial position rather than a crowded axial position.
1,3-diaxial interaction - 2 other hydrogens occupyin gaxial position on same side of ring interacts with side group
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Dangerous Conversationalist
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Dangerous Conversationalist
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Posted: Wed Jul 17, 2013 3:44 am
Combustion Rxn of Hydrocarbons
Hydrocarbons are completely converted to CO2 and water, liberating energy stored in C-H and C-C bonds. EXOTHERMIC RXN, only occurs in presence of molecular oxygen O2, and high temp.
CxHy + (x + y/4)O2 ---> xCO2 + y/2 H2O
Free Radical Halogenation of Alkanes
Reaction that proceeds by a multi-step mechanism including initiation, propagation, termpination, and somtimes inhibition by oxygem
Initiation - A free radical rxn can be initiated by LIGHT or HEAT.
Light: photon (E=hv) collides with a molecule causing homolytic cleavage of a bond, forms 2 radicals
- In homolytic bond cleavage, one e- goes with each atom of bond being cleaved
R-X --> R* + X*
- In heterolytic bond cleavage, both e- go with only 1 of hte atoms
R-X --> R+ + X-
Propagation - Halogen radical collides with alkane molecule (R-H) causing homolytic cleavage of C-H bond with formation of a molecule of hydrogeh halide (H-X) and alkyl radical (R*)
Alkyl halid then formed. Happens by collision of alkyl radical (R*) with molecular halogen (X-X). Results in homolytic cleavage of hte molecular halogen so that a molecule of alkyl halide (R-X) prodcut is formed and molecule of halogen radical (X*) is made.
X* then proceeds to collide iwth another alkane molecule, making H-X alkyl halide molecule and an alkyl radical (R*). Alkyl radical in turn reacts iwth another molecule of molecular halogen (X-X) forming alkyl halide (R-X) and another halogen radical (X*) which reacts with another (R-H) molecule to continue propagation of hte chain reaction
Termination - Chain reaction will continue until one of hte reactive radicals of the propagation steps combines with another radical.
i.e. Combination of 2 halogen radicals (X*) to form a molecule of (X-X), or combination of 2 alkyl radicals (R*) to form molecule of alkane (R-R), or combination of any alkyl radican (R*) with halogen radical (X*) to form alkyl halide (R-X).
Inhibition - Free radical halogenation rxn is inhibited by oxygen. Alkyl radical reacts with a molecule of oxygen to make alkyl peroxy radical (R-O-O*)
Mechanism
INITIATION: (makes radical)
Cl-Cl ---(uv light/hv/heat)--> 2Cl*
PROPAGATION
1)R-H + *Cl ---> R* + HCl (Cl* radical used)
2) R* + Cl-Cl --> R-Cl + Cl* (Cl*radical regenerated)
rinse and repeat
TERMINATION: (remove radical)
a) Cl* + Cl* --> X-X
b) R+ + R* --> R-R
c) R* + X* --> R-X
INHIBITION
R* + O2 --> R-O-O*
This slows down rxn by reducing amount of reactive radical intermediates.
Stereochemsitry of Free Radical Halogenation
After removing hydrogen off R and making hte radical, when Cl* or Br* radical combines it can be on either side of plane of sp2 (removign H makes it go from sp3 --> sp2), so will get a RACEMIC MIXTURE of S and R enantiomers
Stability of alkyl radicals
Alkyl substituents on carbon increases relative stability of radical.
Bromination is more selective than chlorination; both will pick tertiary hydrogen more than on a carbon with all 3H's
Selectivity = reactivity / # of sites available to react
Major product will have the halogen on the tertiary hydrogen - this is mainly true for Br, since it is low energy
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Posted: Wed Jul 17, 2013 11:03 am
Alcohols
n-butanol vs diethyl ether
They are the same formula, but boiling point for n-butanol is much higher than diethyl ether, why?
- n-butanol can form intermolecular hydrogen bonds, and diethyl ether CANNOT; it has O-H group, which gives oxygen partial negative charge, and H attached to it partial positive. In diethyl ether, has an oxygen with non-bonding electrons but all hydrogen atoms are bound to carbons. The C next to O is thus partial positive, and hte H attached to C cannot participate in H-bonding. Therefore, no intermolecular interactions can occur
Basically, alcohols will have higher boiling point than O bound to carbons.
C-OH > C-O-C, C-Br
The more OH, the more higher the boiling point
Molecules with greater mass will have higher bp also (i.e. Bromopentane > methylpentane)
Boiling points for ortho and para phenols
Para-nitrophenol vs ortho-nitrophenol
hydrogen bonding can occur with both the nitro and hydroxyl groups in para-nitrophenol, all H-bonding takes place b/w individual molecules of para- this increases BP; INTERMOLECULAR H - BONDING
For ortho-nitrophenol, on one molecule the nitro and hydroxyl group are close so that intramolecular hydrogen bonding can occur ON THE SAME MOLECULE. INTRAMOLECULAR H-BONDING. This will decrease amt of intermolecular h-bondings with other molecules, decreasing BP
para > ortho
Acidity of Alcohols
Pretty acidic since oxygen is electronegative, very easy to rip the electron from the H making it H+ and O-
--Phenols are more acidic than alcohols because phenoxide ion can be stabilized by resonance, so they are even more chill with the extra electron.
Electron-withdrawing substituents on phenols increase acidity
i.e. para-nitrolphenol; nitro group is a strong e- withdrawing group, stabilizes phenoxide ion thru resonance stabilization. When para-nitrophenol is deprotonated, it can stabilize by withdrawing electrons through delocalized pi system/resonance stability
Electron-donating groups destabilize phenoxide ion and decrease acidity
Ex of EWG: -O-N+=O
Ex of EDG: O-CH3
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Dangerous Conversationalist
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Dangerous Conversationalist
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Posted: Wed Jul 17, 2013 12:03 pm
SN2 Reactions
Alcohol + strong acid = Acid ion replaces hydroxyl group
butalol + H-Br --> butyl bromide + H2O
Rate of reaction for SN2 reactions is a function of 2 variables; BIMOLECULAR
The rate of rxn depends on the concentrations of both the nucleophile and hte electrophile.
rxn rate = k[nucleophile][electrophile]
Reactivity of R-OH: CH2-OH > primary > secondary (because of steric hindrance)
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Posted: Wed Jul 17, 2013 12:37 pm
Carbocation stability
tertiary carbocation is most stable
+C-R3 (tertiary carbocation most stable) > +C-R2H (secondary) > +C-RHH (primary) > methyl CH3+ (least stable)
Stability of carbocations is result of e- donating ability of alkyl groups
SN1 Reactions
SN1 substitution of alcohols occurs when alcohol is treated with strong acid and a neutral water molecule leaves. It happens in 2 steps instead of 1 like SN2.
Step1: planar carbocation when the leaving group dissociates.
Step2: Racemization occurs as the nucleophilic bromide ion attacks equally on either side of carbocation
Tertiary alcohol RRR-C-OH + HBr --> RRR-C-H2O+ ---H2O--> RRR-C+ Br- will now attack both sides of carbocation
--> (R) RRR-C-Br + (S) RRR-C-Br
Unlike SN2 rxn where rate of rxn was a function of 2 variables, SN1 rxn rate is a function of only one variable-- UNIMOLECULAR.
Rate of SN1 depends only on concentration of ELECTROPHILE (species that loses the leaving group over the course of rxn)
rxn rate = k[electrophile] (concentration of ROH)
Reactivity of R-OH: tertiary > secondary > primary (because of stabilization of carbocation)
Rearagement is possible
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Dangerous Conversationalist
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Dangerous Conversationalist
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Posted: Wed Jul 17, 2013 3:11 pm
SN2 vs SN1?
If R-OH is closer to tertiary (RRR-OH), it is SN1, if it's a methyl or R-OH (primary) then it is SN2
SN1 solvent: favors protic polar solvents (i.e. (H-O-H water)
SN2 solvent: favors polar aprotic solvents (i.e. Ether R-O-R; no hydrogens)
Both polar cuz we want a 'charged' environment so they can dissociate
SN2 favors aprotic solvents because if there is protons in solution, nucleophile will never wanna attach to partial positive C behind and kick Br out, why? Because a H+ in the solvent would want the nucleophile more!
For SN1 reactions, protic solvents is ideal so the Leaving group doesn't just stay as a negative charge, will want to bind or give its electron to someone, if solvent is full of H+, will desire to bind to that and creates a stable environment! smile
Elimination rxn of alcohols: dehydration
Involve loss of a molecule of water to form an alkene
Requires strong acid and is favored by high temperatures
tert-butyl alcohol RRR-OH --H2SO4 + heat---> C=C-RR isobutylene + H2O
Step1: protonation of oxygen converts poor leaving group (-OH) into good leaving group (-OH2+)
Step2: oxygen departs iwth its electrons as a neutral water molecule and leaves behind carbocation. Under high temp, the most thermodynamically stable product of this free carbocation is the elimination product, an alkene
Weak base (-OH) removes a proton from a carbon atom adjacent to the carbon bearing the positive charge and the electrons are used to satisfy the positively-charged carbon by forming C-C pi bond
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