Chapters 5-7 Flashcards
Define isomers
same molecular formula
Define constitutional isomers
same molecular formula, but different atom connectivity
Define stereoisomers
same molecular formula, but a different arrangement of atoms/groups in space
Define diastereomers
stereoisomers that are not mirror images
Define enantiomers
stereoisomers are non-superimposable mirror images
What is the chiraliy on a pair of enantiomers?
they have opposite configurations on all of the chiral centers
What is the symmetry for chiral and achiral molecules?
Chiral molecules lack a plane of symmetry
Achiral molecules have at least one plane of symmetry
What are the basic guidlines for the nomenclature of enantiomers?
Assign first priority numbers to groups or atoms attached to the chiral centers
Priority numbers are given based on the atomic numbers
(1=highest, 4-=lowest)
If your #4 points back, you are ready to trace the path from 1 to 2 to 3
If that path is clockwise, it is R
If that path is counterclockwise, it is S
Each R or S is for each chiral center, not the entire molecule
What happens when you are assigning priority numbers for the nomenclature of enantiomers and the atom connected to the carbon is the same?
If the atom is the same, keep moving to the next one connected to it
What happens when you are assigning priority numbers for the nomenclature of enantiomers and you encounter a multiple bond?
They are treated as equal numbers of single bonded atoms
If you have a C=C, break it into two single -C bonds on each carbon
Ex:If you have an aldehyde, make carbon have two single bonds to oxygen, and oxygen have two single bonds to carbon
When happens when you are assigning priority numbers and your #4 is not pointing back?
You need to do two group exchanges to make it go back Two exchanges (on the same chiral center) creates an identical molecule One would create the enantiomer
How do you know the maximum number of stereoisomers for compounds with more than one chiral center?
the maximum (can be fewer) number of stereoisomers is equal to 2^n N is equal to the number of chiral centers
Define meso compounds
These have two or more chiral centers, and at least one plane of symmetry
Relate meso compounds to chirality
All meso compounds/molecules are achiral (plane), but not all achiral are meso
Describe fischer projections
Representations of the molecules in 2D
Whatever is horizontal points toward you
Whatever is vertical is pointing back
What are the general properties of enantiomers?
They have the same physical properties (mp, bp, etc)
They rotate the plane of polarized light by the same extent in different directions
They also have vastly different biological properties
Relate chirality to optically active/inactive
All chiral molecules are optically active
All achiral molecules are optically inactive
Explain the equation [a]D25 = 1/LC
A is the angle of rotation
C is the concentration of the chiral solution
L is the length of tube
The whole thing is called the specific rotation
Define dextrorotatory and levorotatory
Dextrorotatory = positive rotation (rotate to the right)
Levorotatory= negative rotation (rotate to the left)
The +/- has nothing to do with R/S
Define racemic mixture
An equimolar mixture of two enantiomers will be optically inactive, because they will cancel each other out
Describe the general properties of diastereomers
these do not have the same physical properties
How do you determine stereoisomerism in cyclic molecules?
ring flips mean that it is safe to assume that is is flat when you are looking for a plane of symmetry . When you are drawing it flat, make sure you keep the cis-trans properties.
What is thermodynamics about?
where the equilibrium is
What is kinetics about?
how fast the reaction takes place
Define ‘mechanism’
a sequence of steps going from the reactants to the products
Define ‘ionic reactions’
Involve ions or species with partial charges
Define ‘radical reactions’
Involve radicals (species with unpaired electrons)
Define ‘pericyclic reactions’
Have a cyclic transition state
Do not involve ions or radicals
If you wanted to break a halogen bond, which halogen would be easiest?
The larger the halogen, the longer the bond, and the easier it is to break
Describe nucleophiles, include charges
Nucleophiles
Love positive centers
Have an unshared pair of electrons or a negative charge
Can be anionic or neutral
Describe electrophiles, include charges
Electrophile Loves electrons Has a positive charge or a partial positive charge May have a polar covalent bond Ex: Carbonyl groups
Describe leaving groups and their general characteristics
Leaving groups
In this case, the halogen that gets displaced in a nucleophilic substitution
The substituent must be able to leave as a relatively stable, weakly basic molecule or ion
What is special about OTs and OMs?
-OTs and -OMs have many resonance structures, so they are more stable as leaving groups
Describe alkyl halides (or haloalkanes)
Carbon is sp3
Can be 1, 2, or 3*
Describe vinyl halides
Carbon is sp2, due to the double bond
Describe aryl halides
Carbon is sp2, due to the presence of a benzene or other aromatic ring
Describe acetylenic halides (or alkynyl halides)
Carbon is sp, due to the triple bond
Which type of halide will undergo nucleophilic subsitutions and elimination reactions? Why?
Alkyl halides will undergo nucleophilic substitutions and elimination reactions, the others will not
They have sp2 or sp, and the nucleophile cannot approach as well
Describe how SN2 reactions get their name
Sn2 has a bimolecular rate determining step
Second order reaction, Rate=k[nucleophile][substrate]
Describe the process of an SN2 reaction
Nucleophile attacks on the opposite side to the halogen
Halogen departs on the opposite side to where the nucleophile attacks
The configuration of the stereocenter on the carbon inverts
If it has a chirality center that is R, it will become S and vice versa
What kind of nucleophiles do SN2 reactions need (3 characteristics)?
strong, small, polarizable
Describe how SN1 reactions get their name
has a unimolecular rate determining step
Rate depends only on the concentration of the alkyl halide (Rate=k1[RX])
Describe the process of an SN1 reaction
Will have several, typically 3 steps
Breaking C-X bond (ionization of alkyl halide)
Attaching the nucleophile to the carbocation
Making it neutral
Describe the free energy diagram of an SN1 reaction
Three hills, corresponding to each transition state
Each hill is smaller than the next
Two valleys, corresponding to the two intermediates
The jumps from each valley to the next hill is much smaller in the 2nd and 3rd step
Describe the involvement of stereochemistry in an SN1 reaction
The trigonal planar molecule formed by the first step can be attacked from either side
This will form either an R or S compound
If the products do not have a chirality center, they will be the same
What kind of nucleophiles do SN1 reactions want?
weaker
What types of substrates will undergo SN1 reactions? Why?
3* and 2* alkyl halides, The more stable the carbocation formed, the faster the reaction, You are more stable when you have electron donating groups (like R)
What types of substrates will undergo SN2 reactions? Why?
Methyl, 1, and 2 alkyl halides because R groups are blocking the nucleophile from coming in to replace the X
Describe how nucleophilcity is determined in SN2 reactions
Nucleophilicity is measured in terms of the relative rate of its Sn2 reaction
When the nucleophilic atom is the same, it parallels basicities
A good base is usually a good Nu- (unless they are bulky) because both are donating electrons
When the nucleophilic atom is different, it depends on the solvent
What kind of solvents do SN1 reactions like? Why?
Polar protic solvents will stabilize the development of the polar transition state
It stabilizes both the positive and negative sections
What kind of solvents to SN2 reactions like? Why? Give two examples.
The best solvents are polar aprotic (strong dipoles, but without OH or NH)
Ex: DMSO, DMF
They tend to solvate the metal cation the nucleophilic anion is attached to, making the electron pair more available and the approach to the substrate easier
For reactions in aprotic solvents, how do you determine nucleophilicity? Why?
In aprotic solvents, nucleophilicity parallels basicity (F>Cl>Br>I) because only the cations are solvated. F is smaller in size (so has a large charge per surface area) =stronger
For SN2 reactions in protic solvents, how do you determine nucleophilicity? Why?
In protic solvents, nucleophilicity goes I>Br>Cl>F because it follows the level of solvation
The smaller the atom, the more solvated it is
What are some (3) characteristics of good leaving groups?
Small neutral molecules
Those with a lot of resonance
Larger negative molecules (disperse the charge)
Describe how you will name alkenes as E and Z
Examine the 2 groups or atoms attached to each C of the double bond and decide which has higher priority based on the rules used for R and S nomenclature.
Look at each C separately, and choose one from each C
If the highest priority groups are on the same side, it is Z
If the highest priority groups are on different sides, it is E
Describe how the stability of alkenes is determined
The more highly subsituted the alkene is, the more stable it is
tetra-substituted>tri->trans-di>cis-di>mono->unsubstituted
Describe the substrate used for dehydrohalogenation reactions
Any alkyl halide, you remove an X and a H from an adjacent carbon
Describe the mechanism of dehydrohalogenation
Because you are removing an acid, this only occurs with a strong base (OH-, RO-, H-, NH2-, or C-)
This is a one-step reaction, like Sn2, but with an elimination instead of a substitution
The base grabs the H, and the electrons move to the C-C bond, which kicks out the X
The products are BH, an alkene, and X-
The rate is k[base][substrate]
This is called an E2 reaction
Describe how the two reactants in dehydrohalogenation interact
The base has a lone pair, and so does the X, so it needs to come from the opposite side, as in Sn2 reactions
The difference is that the base is attacking the H, not a C
The H and X should be anti coplanar for a fast reaction
Describe the substrate used for dehydration reactions
An alcohol, you remove an OH and a H from an adjacent carbon
Describe the mechanism of dehydration
The first step is to add an strong acid, like H2SO4 or H3PO4
This will protonate the O from the OH, to form water, a good leaving group
The delta plus on the acid H goes to the lone pair on the oxygen
In the next step, the negative ion leftover from the acid grabs the H to form the double bond
This will give you your acid back, so the acid acts as a catalyst
How does the speed of a dehydration reaction vary with the type of substrate?
The speed of reaction with a certain alcohol goes R3COH(3)>R2CHOH (2)>RCH2OH (1*)
You could say that you are forming the most stable alkene (highest number of substituents) or that your RDS forms a more stable carbocation (3>2>1, donate e-)
Describe how carbocations can rearrange themselves
These are called 1,2-shifts of either hydrogen or alkyl groups
Whenever you are working with a cyclic carbocation, make sure that your rearrangement is the most stable both in terms of 3/2/1 prime and in terms of the highest ring (7+ are less stable)
Also make sure that whatever you are doing actually moves the positive charge
What kinds (2) of bases will E2 reactions tolerate that substitution reactions will not?
Bulky, non-polar/weakly polar
What kind of reactant is needed for an E1 reaction that does not occur with a dehydration?
a neutral nucleophile
What solvents do E1 and E2 reactions like?
polar protic (stablize carbocation) and polar aprotic (solvate cation, if any)
Compare the temperatures preferred by substitution or elimination reactions
Elimination reactions do not mind really high temperatures
If an E2 reaction could make two products, how you decide which is the major?
choose more substituted unless working with (CH3)3COK
Describe the synthesis of alkynes from vicinal dihalides
Two halogens on adjacent carbons
Do a double dehydrohalogenation (two E2 reactions)
Describe the synthesis of alkynes from genimal dihalides
Two halogens on the same carbon
Do a double dehydrohalogenation (two E2 reactions)
Describe the hydrogenation of alkenes with either option of reactant
Add H2 in the presence of a metal catalyst (such as Pt, Pd, Ni, or Ru)
This is a syn-addition as both hydrogens are delivered on the same side
If the same reaction happens with D2, you will have the exact same product, except D replaces the H
Describe the hydrogenation of an alkyne into an alkane
Use two H2 instead of one, with the same metal catalysts (Pt, Pd, Ni, Ru)
Describe how you would hydrogenate an alkyne into a cis alkene
Use the same mechanism (RC≡CR + H2), but use a less reactive catalyst such as Pd/CaCO3 or Ni2B/CaCO3
Describe how you would hydrogenate an alkyne into trans
Add Na/Li and NH3/EtNH2 to RC≡CR
Describe how you would transform a terminal alkyne into a regular alkyne
Use NaNH2 then CH3Br, twice
The NaNH2 will rip off a hydrogen, leaving a carbon that is triple bonded with a lone pair
The CH3Br will add back a CH3 to the lone pair
If you have hydrogen and a catalyst, what kinds of reactions will take place in a hydrogenation?
A H2 and a catalyst is always a syn addition
Order the following compounds in order from highest to lowest acidity (or lowest to highest basicity of their conjugates): H-OH, H-OR, H-NH2, , H-C(triple bond)CR , H-CH=CH2, H-CH2CH3
H-OH H-OR H-C(triple bond)CR H-NH2 H-CH=CH2 H-CH2CH3
What is the role of NaNH2 as a reactant?
If a terminal triple bond is present, it will remove the hydrogen from the end carbon, leaving behind a lone pair of electrons
If a halogen is present, it will remove the halogen and an hydrogen, increasing the bond number by one (ex: turn a single bond into a double bond)
What is the role of CH3Br as a reactant in a reaction?
If a lone pair is present, it will add a CH3 there
What is the role of CH3CH2Br as a reactant in a reaction?
If a lone pair is present, it will add a CH3CH2 there
What is the role of secondary or tertary halides as a reactant in a reaction?
If larger molecules, such as R2CHX are used, you will get RC≡CH and R=CHR (or R=CR2, for a tertiary alkyl halide
What is the role of Li/NH3 as a reactant in a reaction?
This will turn an RC≡CR into a trans RHC=CHR
What is the role of H2/Pd/CaCO3 as a reactant in a reaction?
This will turn an alkyne into a cis alkene
What is the role of H2/Pd as a reactant in a reaction?
This will turn an alkyne/alkene into an alkane
