week 4: alkenes (1 & 2) Flashcards
what is saturation?
for a molecule to be saturated, it has to contain all the hydrogens it CAN contain
what is unsaturation?
(of a hydrocarbon)
a hydrocarbon is unsaturated if its number of hydrogens is less than in an open-chain alkane with the same number of carbon atoms
in comparison to an alkane, the presence of what makes a molecule unsaturated?
- the presence of a double bond (removes 2 H)
- the presence of a ring (removes 2 H)
how do you work out the degrees of unsaturation (when given the chemical structure of a compound)?
- count the number of double bonds (each = 1 degree)
- count the number of rings
(each = 1 degree) - double bonds + rings
a triple bond counts as __ degrees of saturation
2
how do you work out the degrees of unsaturation (when ONLY given the chemical formula of a compound)? list the 4 cases
(1) pure hydrocarbon:
compare to fully saturated alkane with SAME number of carbons
(2) halogen atom(s) present:
- halogens replace H atoms
- add the number of halogen atoms to the number of H atoms
- compare to alkane with same number of carbons
(3) oxygen atom(s) present:
- insertion of an O atom does NOT change the number of H atoms
- H stays the same, compare to alkane with same number of carbons
(4) nitrogen atom(s) present:
- insertion of a N atom increases the number of H atoms by 1
- subtract one H for each N atom present
- compare to alkane with same number of carbons
what do we do after obtaining the hydrocarbon equivalent formula of unsaturated hydrocarbons?
- compare with fully saturated alkane formula
- find the difference in H atoms
- divide the difference by 2 to obtain degrees of unsaturation
(each degree = loss of 2 H)
what is the relationship between molecule stability, its chemical reactivity, and its overall strain?
the more unstable a molecule is, the more chemically reactive it is, the higher strain it has
which alkene is more stable? cis- or trans- isomers? which has more steric stain?
- trans- isomers are more stable, less reactive, and have no steric strain
- cis-isomers have more steric strain
what is responsible for energy differences among disubstituted alkenes?
steric repulsion (or steric strain)
how does steric strain compare in different alkane isomers?
- highest strain - when 2 big groups are connected to same carbon
- mid strain - when 2 big groups on separate carbons (CIS ISOMER)
- lowest strain - TRANS ISOMER
what is a nucleophile?
- a molecule that has a high electron density
- is attracted to positive charge
- reacts with electrophiles
why are alkenes considered nucleophiles?
the π bond in alkenes make them electron-rich (negatively charged), therefore nucleophiles
what are attracted to alkenes?
electrophiles (Lewis acids)
what is electrophilic addition?
a reaction where we add unit X–Y across the double bond of an alkene
what are the steps of electrophilic addition to an alkene?
(1) the π (pi) bond breaks
(2) one alkene carbon gets the X, the other gets the Y of the electrophile
in electrophilic addition reactions of HX (where X is a halogen), which reactions are more successful?
more likely to occur:
- HCl - used in gaseous form
- HBr - used in gaseous form
as for the rest:
- HF - too reactive
- HI - too unreactive (use KI in presence of H3PO4 (phosphoric acid) to form reaction)
what non polar solvent is usually used to dissolve non polar alkenes in solution?
CCl4 (carbon tetrachloride)
during electrophilic addition, what is the reaction intermediate called? describe it
- carbocation
- sp^2 hybridized
- trigonal planar geometry
- empty 2p atomic orbital
what type of reaction is electrophilic addition? why?
- an exergonic, spontaneous reaction
why?
- since the first step involves breaking a pi bond, it requires energy, (the slow step)
- the second step involves the attack of a fully negative particle (ex: Br-) on the fully positive carbocation
- according to Coulomb’s law, that large electrostatic attraction makes step 2 (fast step)
- first slow, second fast = overall exergonic (-ΔG) & spontaneous
what are the different types of electrophilic addition reactions to alkenes?
- orientation of electrophilic additions: Markovnikov’s rule (regiospecific)
- addition of halogens (stereospecific)
- addition of water
- hydrogenation (adding H2)
- oxidation (adding O)
what does it mean to be regiospecifc?
atoms are oriented in specific positions (ex: Markovnikov’s rule)
what does Markovnikov’s rule say?
in reactions of HX to an alkene, the H atom is captured by the alkene carbon attached to MOST hydrogens, and the X atom is captured by the most highly SUBSTITUTED alkene carbon (i.e the other carbon)
which has a more stable carbocation, a methyl group (-CH3), primary, secondary, or tertiary carbocation? why?
- a tertiary carbocation is most stable
why?
- a methyl group donates negative electron density, reducing the positive charge on the carbocation
- however, in the tertiary carbocation, there are THREE electron-donating methyl groups, so the positive charge is much more reduced
- less positive charge = more stability
will a primary or a tertiary carbocation produce more product after undergoing electrophilic addition? why?
- tertiary carbocation
why?
- even tho a primary carbocation is more reactive (bc more unstable) it has a higher activation energy
- therefore, the first step is faster with a tertiary carbocation, and will form more product first
since elemental halogens (X2) are symmetrical (non polar), how can electrophilic addition take place?
- through an induced-dipole
how? - when for example, a Br2 molecule is far away from the alkene, it remains non polar
- when it approaches the electron-rich pi bond, its electrons are repelled, leaving one Br atom δ+, and the more distant Br atom δ- (creating an induced-dipole)
- the bromine molecules become polarized
what does it mean to be stereospecific?
atoms go in particular 3-dimensional orientations (ex: halogens added to alkenes)
explain how the addition of halogens to alkenes is a stereospecific reaction?
- when adding Br2 to an alkene ring, we will have a trans molecule
- this is because halogens are big molecules, and if they were captured on the same side, they would repel each other
- the driving force of this is steric strain - (there will less steric strain in a trans form)
what is the addition of water to alkenes reaction called? what does it produce?
- reaction is called hydration
- forms alcohols
what compounds will NOT get reduced by hydrogenation?
- benzene rings
- carbonyl groups
what are the different methods of adding water to alkenes (or hydration reactions)?
(1) brute-force (Markovnikov addition)
(2) oxymercuration (Markovnikov addition)
(3) hydroboration (anti-Markovnikov addition)
describe brute-force reactions
- typical Markovnikov rules, where you add the H to the carbon attached to most hydrogens, and the OH to the other carbon
- the reaction is acid-catalyzed because the H+ atom from the acid is used to break the pi-bond
what problems do brute-force reactions have?
- highly acidic conditions (50% sulfuric acid)
- need high temperatures
- NOT ideal for research/undergrad lab environments
what are the steps of oxymercuration? does it follow Markovnikov or anti-Markovnikov addition?
- step 1: treatment with mercuric acetate ( Hg(OAc)2 )
- step 2: treatment with sodium borohydride (NaBH4)
- follows Markovnikov addition
what are the steps of hydroboration? does it follow Markovnikov or anti-Markovnikov addition?
- step 1: addition of borane (BH3) to diethyl ether (Et2O)
- step 2: addition of H2O2 in NaOH solution
- follows anti-Markovnikov addition
what is meant by syn-addition in hydroboration?
if we have a ring alkene, the -H and -OH are added on the same side of the ring (cis)
what type of reaction is alkene hydrogenation?
a reduction
