Exam 2 (Ch 3, 5, 14, 15) Flashcards
what is a bronsted acid
proton donor
what is a bronsted base
proton acceptor
what does a bronsted acid look like
H attached to EN atom
what can a bronsted acid do?
stabilize a negative charge
what do you look for when looking for bronsted bases?
a negative charge and localized lone pairs
what is a conjugate acid
a protonated base
what is a conjugate base
a deprotonated acid
what do strong bases form?
weak conjugate acids
what do strong acids form?
weak conjugate bases
why do strong acids form weak conjugate bases?
because they really want to give up that proton, don’t want it back
why do strong bases form weak conjugate acids?
because they really want to take a proton, don’t want to give it up once they get it
what does the equilibrium of an acid-base reactio always favor?
the side with the higher, more positive pka value (weaker acid and weaker base)
when using ARIO to analyze the strength of an acid or determine equilibrium, what are you comparing?
the stability of the conjugate base
what is the ability to stabilize a negative charge proportional to?
the stability of a conjugate base which leads to a stronger acid!
what does ARIO stand for?
Atom: electronegativity and size
Resonance
Induction
Orbital
which is better able to stabilize a negative charge, resulting in a more stable conjugate base and a stronger acid, sp or sp3 hybridized orbital? why?
sp hybridized is better because more s character holds negative charge closer to nucleus = more stable
when identifying the most acidic hydrogen in a compound, what 3 atoms do you look for attached hydrogens on first? how do you find if those 3 are not present?
Sulfur, Oxygen, and Nitrogen; if not present, look for resonance, induction, or hybridized orbitals
define constitutional isomers
same molecular formula, different connectivity
define stereoisomers
same MF, same connectivity, different spatial arrangement
what are the 3 types of stereoisomers?
- ring
- double bond
- chirality stereoisomers
how do you assign configuration for ring stereoisomers?
cis: all wedges or all dashes, groups on same side
trans: wedge and dash mixed up, groups on different sides
is there any rotation in ring stereisomers? why or why not?
no rotation, bonds locked into place
how do you assign configuration for double bond stereoisomers?
cis: 2 identical groups on SAME side of double bond
trans: 2 identical groups on OPPOSITE side of double bond
are double bonds rotatable? why or why not?
no rotation, P orbitals must remain parallel so pi bond doesn’t break
how do you assign configuration of 2 identical groups are on the same END of a double bond? is cis/trans stereoisomerism possible here?
no cis/trans!
Z: 2 higher priority groups on the same side of DB (like cis)
E: 2 higher priority groups on opposite side of DB (like trans)
what determines higher priority?
larger atomic number
how do you assign R/S configuration of chiral centers?
- assign priority to each of the 4 groups
- make sure lowest priority is on a dash!!
- draw path from 1-2-3
clockwise: (R)
counterclockwise: (S)
how do you assign D/L configuration for Fischer projections?
analyze bottom-most carbohydrate
OH on Left: L
OH on right: D
do chiral molecules have an optical rotation value?
yes!
what do chiral molecules possess?
non super-imposable mirror images
what is an enantiomer?
a chiral compound’s one non super-imposable mirror image
when do chiral molecules exist?
when a carbon is attached to 4 DIFFERENT groups
what is a racemic mixture?
a 50/50 mixture or both enantiomers of a compound (usually costs too much to separate. so lump all together and you’re fine)
what are the only 2 differences between enantiomers?
- optical rotation of each
2. how each interacts with other molecules (smell and thalidimide babies)
what are diastereomers?
non super-imposable and also NOT mirror images
which can be distinguised by chemical properties: enantiomers or diastereomers?
diastereomers
what are meso compounds?
possess a chiral center but also have a plane of symmetry, so ACHIRAL
do meso compounds have optical rotation?
nope!
what does NMR stand for?
Nuclear Magnetic Resonance
what does NMR mean?
nucleus of an atom is in a magnetic field and it resonates by absorbing radiofrequency
what kind of nuclei can spin? (2)
- odd atomic number
- odd mass number
just need to have one
what is it called when a nucleus can spin?
NMR active
what is the most commonly observed NMR nucleus?
hydrogen
how many allowed spin states does hydrogen have?
2
what is Bo?
an external magnetic field
in the presence of Bo, describe the 2 spin states of hydrogen
+1/2: the alpa spin state, lower in energy because flows with current of Bo
-1/2: the beta spin state, higher in energy because flows against current of Bo
what increases as Bo increases?
the energy gap (delta E) between alpha and beta spin states
how do you calculate number of allowed spin states?
2I + 1
what does the energy of radiofrequency match?
the change in energy between alpha and beta spin states
what can the energy of radiofrequency do? what is this called?
spin flip! excite a nucleus from alpha to beta spin state
in what state is the nucleus during/after a spin flip from radiofrequency energy?
resonating/in resonance
define shielding
electrons create opposing magnetic field that partially cancels out the external magnetic field (Bo)
what does shielding do to the felt Bo, the change in energy, and the radiofrequency?
- decreases felt Bo by nucleus
- decreased change in energy
- decreases the radiofrequency energy required to excite a nucleus from alpha to beta spin state
define deshielding
withdrawal of electron density via induction from electronegative atoms
what does deshielding do to the felt Bo, the change in energy, and the radiofrequency?
- increases the felt Bo
- increases change in energy
- increases the radiofrequency energy required to excite a nucleus from alpha to beta spin state
what is anisotropy?
when the electrons in pi bonds create magnetic fields that either oppose (shield) or reinforce (deshield) the external magnetic field (Bo)
name 3 functional groups where anisotropy can occur
- alkenes
2 carbonyls - benzene rings
what does anistropy explain?
the large deshielded chemical shift of benzenes and aldehydes
what allows NMR differentiation?
the different electronic environments of each hydrogen nucleus in a compound
what is splitting?
when NMR peaks appear anywhere from a singlet to a dectet, caused by the small magnetic field created by the neighboring hydrogen atoms that either shieldor deshield Bo
what is integration?
the amount of hydrogen atoms that produce an NMR signal
how to you determine splitting pattern?
N+1 (neighboring H atoms +1)
what is the most common molecular weight of Cl in NMR?
35
what is the most common molecular weight of Br in NMR?
79
what does a quartet usually indicate structurally?
a H next to CH3
what does 3H usually indicate structurally?
methyl (CH3)
what does 2H usually indicate structurally?
CH2 group
what does 6H usually indicate structurally?
2 identical CH3 groups
