as carbons are added in a single chain and molecular weight increases, the intermolecular forces increase and, thus, the boiling point of the alkane increases.
branching significantly lowers the boiling point.
melting point of unbranched alkanes tends to increase with increasing molecular weight, though not as smoothly (crystals depend on shape and size)
alkane properties picture
chair conformations and energy
large substituents in the axial position require more energy and create less stability.
five-carbon rings and less
the cis isomers are meso compounds. also compounds that exist in equilibrium with its own mirror image are also optically inactive.
once begun, generates its own heat and is self-perpetuating. is a radical reaction.
heat of combustion
the change in enthalpy of a combustion reaction. can be used to compare relative stability of isomers. the higher the heat of combustion, the higher the energy level of the molecule, the less stable the molecule.
3 steps in halogenation
1) initiation 2) propagation 3) termination
the halogen starts as a diatomic molecule. the molecule is homolytically cleaved by heat or by UV light, resulting in free radicals.
the halogen radical removes a hydrogen from the alkane resulting in an alkyl radical. the alkyl radical may now react with a diatomic halogen molecule creating an alkyl halide and a new halogen radical. can continue indefinitely or go to termination.
either two radicals bond or a radical bonds to the wall of the container to end the chain reaction or propagation.
stability of alkyl radicals
3°>2°>1°>methyl exhibit trigonal planer geometry
reactivity of halogens
selectivity of halogens
the tertiary product
will not necessarily be the primary product
undergoes substitution NOT addition. flat molecule. stabilized by resonance. consider electron withdrawing in most situations (except ring)
electron withdrawing group
in the R position, deactivates the ring and directs new subtituents to the meta position
electron donating groups
activate the ring and direct any substituents to the ortho and para positions
exception. electron withdrawing and deactivate the ring as expected, but are ortho-para directors.
thermodynamic stability of alkenes (opposite of reactivity)
physical properties of alkenes
same trends as alkanes. an increase in molecular weight leads to an increase in boiling point. branching decreases boiling point. very slightly soluble in water and have a lower density than water. more acidic than alkanes
synthesis of an alkene
occurs via an elimination reaction. one or two functional groups are eliminated to form a double bond.
dehydration of an alcohol
an E1 reaction where an alcohol forms an alkene in the presence of hot concentrated acid.
the rate depends on the concentration of only one species.
follows the same trend as radical stability
most stable, substituted alkene.
dehydration of an alcohol
states that the major product of elimination will be the most substituted alkene
exothermic, high energy of activation, syn addition.
syn addition of alkynes
creates a cis alkene
oxidation of alkenes
may produce glycols (hydroxyl groups on adjacent carbons) or may cleave teh alkene at the double bond as in ozonolysis.
alkynes produce carboxylic acids when undergoing ozonolysis
ozonolysis of an alkene
important for alkenes.
when hydrogen halides are added to alkenes
they follow Markovnikov's rule.
the hydrogen will add to the least substituted carbon of the double bond. 1) the hydrogen, a Bronsted-Lowery acid, creates a positively charged proton, which acts as the electrophile 2) the newly formed carbocation picks up the negatively charged halide ion. first step determines the rate.
electrophilic addition via Markovnikov's rule
if peroxides (ROOR) are present, the bromine, not the hydrogen, will add to the least substituted carbon. the other halogens still follow Markovnikov's rule even in the presense of peroxides
hydration of an alkene
follows Markovnikov's rule. takes place when water is added to an alkene in the presence of an acid. the reverse of dehydration of an alcohol.
low temperatures and dilute acid drive this reaction toward alcohol formation; high temperatures and concentrated acid drive the reaction toward alkene formation
creates an alcohol from an alkene. two step process which also follows Markovnikov's rule, but barely results in the rearrangement of the carbon.
know that organometallic compounds like to lose electrons and take on a full or partial positive charge
if an alcohol is used instead of water, the corresponding ether is produced.
anti-Markovnikov and syn addition.
halogenation of an alkene
Br₂ and Cl₂ add to alkenes readily via anti-addition to form vic-dihalides (two halogens connected to adjacent carbons).
when halogenation takes place with water, a _____ is formed and Markovnikov's rule is followed where the electrophile adds to the least substituted carbon. water acts as the nucleophile in the second step instead of the bromide ion. ___ is a hydroxyl group and a halogen attached to adjacent carbons.)
occur when one funcitonal group replaces another. Sn1 and Sn2
leaving group determines the rate. if the carbocation carbon began and ended an Sn1 reaction as a chiral carbon, both enantiomers would be produced. the intermediate carbocation is planar and the nucleophile is able to attack it from either side. carbon skeleton rearrangement may occur if the carbocation can rearrange to a more stable form. E1 often accompanies Sn1 reactions because the nucleophile may act as a base to abstract a proton from the carbocation, forming a carbon carbon double bond.
reactions occur in a single step. the rate is dependent on the concentration of the nucleophile and the substrate. the nucleophile attacks the intact substrate from behind the leaving group and knocks the leaving group free while bonding to the substrate. *inverse of configuration on the carbon being attacked by the nucleophile. if the carbon were chiral, the relative configuration would be changed but the absolute configuration may or may not be changed. *tertiary carbon would sterically hinder the nuclephile.
rate of Sn2 reactions
decreases from methyl to secondary substrates. don't typically occur with tertiary substrates. if the nucleophile is a strong base and the substrate is too hindered, E2 reaction may occur.
Sn2 reaction mechanism
a base is always a stronger nucleophile than its conjugate acid, but basicity is not the same thing as nucleophilicity.
if a nucleophile behaves as a base, then elimination occurs. to avoid this, use a less bulky nucleophile.
a negative charge and polarizability add to nucleophilicity.
electronegativity reduces nucleophilicity.
in general, nucleophilicity decreases going up and to the right on the periodic table.
polar protic solvents
(polar solvents that can hydrogen bond) stabilzie the nucleophile and any carbocation that may form. a stable nucleophile slows Sn2 reactions, while a stable carbocation increases the rate of Sn1 reactions.
polar aprotic solvents
(polar solvents that can't form hydrogen bonds) do not form strong bonds with ions and thus increase the rate of Sn2 reactions while inhibiting Sn1 reactions.
boiling the solvent in Sn1
provides energy for the formation of the carbocation
the solvent acts as the nucleophile
best are those that are stable when they leave. generally, the weaker the base, the better the leaving group. electron withdrawing effects and polarizablity also make for a good leaving group. the leaving group will always be more stable than the nucleophile.
Sn2 requires a strong nucleophile, while nucleophilic strength doesn't affect Sn1.
Sn2 reactions don't occur with a sterically hindered SUBSTRATE. Sn2 require a methyl, primary, or secondary substrate, while Sn1 requires a secondary or tertiary substrate.
a highly polar SOLVENT increases the reaction rate of Sn1 by stabilizing the carbocation, but slows down Sn2 reactions by by stabilizing the nucleophile
the SPEED of an Sn2 reaction depends upon the concentration of the substrate and the nucleophile, while the speed of an Sn1 depends on the substrate
Sn2 inverts STEREOCHEMISTRY about the chiral center, while Sn1 creates a racemic mixture
Sn1 may be accompanied by carbon skeleton rearrangement but Sn2 never rearranges the carbon skeleton.
elemination reactions can accompany
both Sn1 and Sn2 reactions. elimination occurs when the nucleophile behaves a a base rather than a nucleophile; it abstracts a proton rather than attacking a carbon. elimination reactions always result in a carbon-carbon double bond. E1 and E2 kinetics are similar to Sn1 and Sn2 respectively.
Physical properties of alcohols
same general trends as alkanes. boiling point goes up with molecular weight and down with branching. melting point trend not as reliable, but goes up with molecular weight. branching has less clear effect on branching.
boiling and melting points are higher relative to alkanes because of hydrogen bonding.
alcohols as acids
less acidic than water. order of acidity from strongest to weakest is methyl>1°>2°>3° methyl groups are electron donating, thus prevent the carbon from absorbing some of the excess negative charge of the conjugate.
synthesis of alcohols
grignard synthesis; reduction synthesis;
Grignard synthesis of an alcohol
grignard reagents will react in similar fashion with C=N, C≡N, S=O, N=O. grignard strong enough to deprotonate O-H, N-H, S-H, -C≡C-H
reduction synthesis of an alcohol
mechanism similar to Grignard, hydrides will react with carbonyls to form alcohols. unlike Grignard, does not extend the carbon skeleton.
in reduction synthesis
both NaBH₄ and LiAlH₄ will reduce aldehydes and ketones, but only LiAlH₄ is strong enough to reduce esters and acetates.
Alcohols as nucleophiles
loss of H₂; addition of O or O₂; addition of X₂ (halogens)
addition of H₂ (or H⁻); loss of O or O₂; loss of X₂
neither oxidation nor reduction
addition or loss of H⁺, H₂O, HX, etc.
oxidation/reduction of alcohols
common oxidizing agents
K₂Cr₂O₇; KMnO₄; H₂CrO₄; O₂; Br₂
LiAlH₄; NaBH₄; H₂ + pressure
alkyl halides from alcohols
Sn1 with tertiary alcohols and Sn2 with others.
note on alkyl halides from alcohols
alcohols are very weak electrophiles because the hydroxyl group is such a a weak leaving group. protonating the hydroxyl group makes the good leaving group water. however, protonating an alcohol requires a strong acid. strong acids react with most good nucleophiles, destroying their nucleophilicity.
can also convert alcohols to alkyl halides. PCl₃, PBr₃, and PI₃, via an Sn2 mechanism resulting in poor yields with tertiary alcohols.
thionyl chloride SOCl₂
another reagent for producing alkyl halides from alcohols. results in sulfur dioxide and hydrochloric acid.
very weak bases and excellent leaving groups. when tosylates and mesylates are leaving groups, the reaction may proceed via an Sn1 or Sn2
formation of sulfonates
retention of configuration (in formation, not when they are leaving groups, which can proceed either Sn1 or Sn2)
a dehydration of an alcohol that results in an unexpected product. when hot sulfuric acid is added to an alcohol, the expected product is an alkene. however, if the alcohol is a vicinal diol, the product will be a ketone or aldehyde.
pinacol rearrangement mechanism
other than epoxides, are relatively nonreactive. roughly as soluble in water as alcohols, yet other organic compounds to be much more soluble in ethers than alcohols because no hydrogen bonds need to be broken. useful solvents boiling point roughly comparable to that of an alkane with similar molecular weight (can't hydrogen bond with itself). low boiling point makes useful solvent.
ether cleaved by halo-acid
either HI or HBr.
react with water in the presence of an acid catalyst to form diols, commonly called glycols.