135A organic - introduction Flashcards
what is the structure of a carbocation
planar 120°, sp2 hybridised, empty p orbital
incomplete octets are allowed in lewis structures e.g. BH3 but what does this mean about the molecule?
highly reactive (BH3 will form B2H6)
what elements can sometimes hold 10-12 electrons even though lewis formula only allows for 8
second row Na—> Ar
What will the charge be on N (P5)
In this diagram N has 4 electrons belonging to it, but it should have 5 in its natural state; has a charge of +1
What will the charge be on C (P4)
In diagram has 4 belonging to it and has 4 in its natural state; will be neutral
Why is a tertiary carbocation stable
hyperconjugation the bonding pair of σ electrons in adjacent C-H bonds can align with the empty p orbital of the carbocation. Carbon adjacent to c+ develops partial positive charge; + charge no longer localised on carbocation
Lewis structure of the methoxide anion
define electronegativity
ability of atoms to attract electrons in a covalent bond towards themselves
how to represent the polar bond of C-F
arrow goes towards most electronegative
how to represent the polar bond of C-Li
arrow goes towards most electronegative
what is the inductive effect
shifting of electrons in bonds in response to electronegativity of nearby atoms
trend in atom size across a period
decreases
if this molecule is drawn with balls and sticks, nitrogen is drawn smaller than boron.
atomic radius decreases across period/ has higher Z effective because more protons but same electron shells/ its electron cloud is smaller
draw molecule polarity diagram for water
lone pairs have the highest electron density, arrows go from high ED to low ED
draw molecule polarity diagram for CO2
define the terms to work out the dipole moment
Why does CF4 have no dipole moment
polar bonds but due to geometry has no positive or negative end
Why does a polar substance not dissolve in a non-polar solvent?
only substances with similar dipole moments will dissolve in each other
If there is a high difference in electronegativity between two atoms is there a large or small degree of covalent character
lower % (more ionic)
equation for pKa
equation for pH
equation for pOH (OH- concentration)
higher pKa = lower/higher Ka = weaker/stronger acid
higher pKa = lower Ka = weaker acid
why is do we usually write H3O+ in equations for dissociation not H+
H+ is very reactive and almost always reacts further with more H2O to make H3O+
why for the Ka of this reaction would you leave H2O out
so little is used up that its concentration is basically constant
what does Ka describe
acidity constant in water at 25°
what does the sum of pH and pOH equal
Kw equation
Kw = [H3O+][OH] = 10-14
what is Kb
basicity constant (strength of base) at 25°
what does a pKa below 1 mean
acid is fully dissociated
what does a pKa above 1 mean
acid partially dissociated
the equilibrium constant always favours the side with the stronger/weaker aid (in an acid base reaction)
weaker acid (equilibrium will lie to right by factor of 10^11 (15-4))
Inverse strength rule- the stronger acid will have a stronger/weaker congruent base
stronger acid = weaker con base
will this be a strong or weak acid
strong acid/ less e- density around the COOH group as electron withdrawing groups (want less e- density because the molecule has to attract a negative species to take H+ close to it)/ more stable anion favours dissociation
will this be a strong or weak acid
weak acid/ more e- density around COOH/ CH3 are electron donating/ disfavours dissociation
Lewis definition of a nucleophile and electrophile ( in terms of electrons)
nucleophile- electron pair donor
electrophile- electron pair acceptor
Bronsted-Lowery definition of a nucleophile and electrophile (in terms of protons)
nucleophile- proton acceptor
electrophile- proton donar
what is the difference between a Bronsted-Lowery proton and a Lewis acid
All Bronsted-Lowery acids must contain a proton
single/double bonds are shorter
double bonds are shorter
two resonance structures for a methanoate ion
what average charge does each oxygen atom have in these resonance structures
-2/3 (-1)+(-1) / 3 atoms = -2/3
are resonane structures real
no- the real molecule is an average of all resonance structures (this hybrid will be lower in energy than either individual resonance form
Why does more resonance structures= more stable
more ways for e- to be distributed/ charge is less localised to one area of the molecule/ more partial charges across more atoms
why are these two not resonance structures
If we move two electrons over, N would have 12 electrons around it (N has a lone pair on it too)
resonance structure of this benzene ring
the actual structure of benzene in a hybrid between the two and just form a delocalised ring due to stability
what charge would each oxygen atom have
(-1)+(-1) / 4 atoms = -0.5
Rules of drawing resonance structures
- atoms must be in the same place
- cannot break octet rule
- have to have the same charge
Are these resonance structures
no- chlorine has moved place
Are these resonance structures
no- the H has moved places
What does the nitrogroup look like
which structure will contribute more to the resonance hybrid
It is the most stable structure/ more stable to have the - on the oxygen atom rather than the carbon because oxygen has higher electronegativity
Why are carboxylic acids stronger acids than alcohols
When the H is removed the COO- anion is much more stable (shared negative charge) than the O- (must take all the negative charge)
what are tautomers
structures in equilibrium with different geometrics and different arrangements of nuclei and electrons
What generally moves in a tautomer
(the lightest atom) Hydrogen
What is the tautomer of this molecule
(ignore arrows)
Where does the curly arrow have to come from in a double bond
the π area as this is the area that donates the electrons
reaction of cyclohexane with ammonia
positive charge on N=nitrogen has 4 electrons belonging to it but 5 in its natural state
is a Lewis acid/base an electron pair donor/acceptor
acid- electron acceptor (donates proton)- electrophile
base- electron donor (accepts proton)- nucleophile
Some things can react as a base or a nucleophile. How is OH- reacting as a base
Lewis base- electron donor/ proton acceptor. This OH- is accepting a proton
Some things can react as a base or a nucleophile. How is OH- reacting as a nucleophile
nucleophile- electron donor. In this case the OH- is donating itself (electrons)
what does basicity and nucleophilicity mean
basicity- affinity towards a proton
nucleophilicity- affinity towards carbon (ones that have a partial or full positive charge)
Here is the OH- acting as a base or nucleophile
nucleophile- it donates electrons (itself)
When drawing mechanism reactions between H+ in solution we do not use H+ as this is very reactive and doesn’t exist for long in a solution; what do we write instead
as H3O+
How would these two molecules react
How would these two molecules react
How would these two molecules react
How would these two molecules react now that there are CH3 groups attached instead of R groups
If there are hydrogens around a strong base (alkyl lithiums), Carbon can act as a base(accepts a proton). Electrons move from the C to form a C=C and this forces the electrons onto O
MO of a C=C bond
show how sp3 hybridisation forms in a CH3 molecule
is OMe electron withdrawing or donating
donating
