Chem Final Review Chapter 5
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study of the 3D structure of molecuels
same bonding sequence, but they differ in the orientation of their atoms in space
Objects with left and right handed forms are __. How do when this property exists?
Nonsuperimposable mirror-image molecules are called __. A __ compound always has an __.
- if hte mirror image is different from the actual object (nonsuperimposable)
- chiral compound
Most common feature of chirality? This feature is the most common ex. of a __, the IUPAC term for any atom holding a set of ligands in a spatial arrangement that is not superimposable on its mirror image.
- carbon bonded to four diferent atoms, creating an asymmetric carbon atom or chiral carbon atom
- chirality center
Chirality centers belong to an even broader group called __. A __ is any atom at which the interchange of two groups gives a stereoisomer.
If there is an internal plane of symmetry, what is it?
Any asymmetric carbon has two possible mirror-image spatial arrangements, which we call __.
achiral, even if it may have chiral centers
assigning R (right) or S (left) to its 3d structure
1) assign priorities to each group
- 2) using models, put the fourth group away from you and view the molecule with the first, second, and third groups radiating twoard you. Draw from 1 to 3
Rotation of the plane of polarized light is __, and substances that rotate the plane of polarized light are said to be __.
Properties of enantiomers?
Directions of light?
- optical activity
- optically active
- have identical physical properties, except for the direction they rotate the plane of polarized light
- dextrorotatory (+ or d): toward the right
- levorotatory (- or l): toward the left
Formula for specific rotation
observed rotation/ c (g/mL) * length in dm
A solution of equal amounts of two enantiomers, so that the mixture is optically inactive, is called a __. Sometimes, its called a __, __, or a __.
When will this kind of mixture be created?
- racemic mixture
when optically inactive reactants are used
the __ is defined as the ratio of its rotation to the rotation of a pure enantiomer.
(observed rotation)/ (rotation of pure enantiomer) x 100%
The __ is a similar method for expressing the relative amounts of enantiomers in a mixture. To comput ethis, we calculate the excess of the predominant enantiomer as a percentage of hte entire mixture. For a chemically pure compound, teh calculation of this generally gives the same result as the calculation of optical purity and we use the terms interchangeably.
When can allene be chiral?
when we add molecules to it, causing its mirror to be different
Rotating a Fischer projection by 180 degrees does what to it?
keeps it the same
Stereoisomers that are not mirror images of one another= __
The biggest example of diastereomers? Other examples?
moelucels with two or more chirality centers
What is the 2^n rule?
a compound with n asymmetric carbon atoms might have as many as 2^n stereoisomers
Compounds that are achiral even though they have asymmetric carbon atoms are called __.
the detailed stereochemical picture of a molecule, including how the atoms are arranged in space. Alternatively, the R or S config at each chiral center
the experimentally determined relationship between the configs of two molecues
Properties of diastereomers
different physical properties
- 1) has a lalogen bonded to one of the sp3 carbon atoms
- 2) halogen atom bonded to one of teh sp2 hybrid carbon atoms of an alkene
- 3) halogen atom bonded to one of the sp2 carbons of an aromatic ring
- 1) halogen atoms bonded to the same carbon atom
- 1) two halogens bonded to adjacent carbon atoms
What are the effects that affect dipole moments?
electronegativity: I < B < Cl < F
bond lengths: CF < CCl < CBr < CI
These oppose each other to lead to an overall result of: CI < CBr < CF < CCl
Two types of intermolecular forces influence the BP of alkyl halides. The __ is the strongest intermolecular attraction in alkyl halides. __ also affect it.
What is the trend in BP?
- London force
- dipole dipole
higher MW have higher BP because heavier and greater surface area
- Alkyl fluorides and chlorieds with just one Cl are less dense than water
- two or more Cl are denser, as well as alkyl Br and Alkyl I
free rad bromination of alkenes can be carried out in a selective manner. Why?
because the bromine will abstract a proton from the allylic position and the most subbed will be alkene is the most stable
Allylic Bromination steps
Initiation: Br absorbs light, forming rads
First Prop: A bromine rad abstracts an allylic H
Second Prop: Either radical carbon can react with Br
What is the reaction with NBS?
NBS + HBr--> Br2 and that other product with the H
In many eliminations, a molecule of HX is lost from the alkyl halide to give an __. These eliminations are called __ because a hydrogen halide has been removed from the alkyl halide.
In a __, a nucleoophile replaces a leaving group from a carbon atom, using its lone pairs of electrons to form a new bond to the arbon atom.
In a __, both the halideion and another substituent are lost. A new pi bond is formed.
- nucleophilic substitution
- an elimination
In the elimination, the reagent reacts as a __.
- how many steps?
- what type of reaction?
- middle structure?
- order of reactivity?
- - one
- - concerted reaction
- - transition state
- - CH3X> primary> secondary (doesn't occur on tertiary)
What can we generalize about bases? Give an example.
A base is always a stronger nucleophile than its conjugate acid- methoxide ion versus methanol ion
Differentiate between base and nucleophile.
if the new bond is to a proton, it has reacted as a base. If the new bond is to a carbon, it has reacted as a nucleophile
decreases from L to R in the periodic table, following the increase in EN from left to right, but increases down the periodic table
Bulky groups on nucleophile?
will hinder back side attack and prevent substitution--> elimination
Good leaving groups should be __.
Rates for Sn2
weak bases (conjugate bases of strong acids)
CH3X > prim > sec >> tert
Stereochemistry of Sn2
stereospecific: inversion of configuation
- - very weak solvent
- 1) formation of carbocation (rate limiting)
- 2) nucleophilic attack on the carbocation
- 3) loss of proton to solvent ( if the nucleophile is water or an alcohol
- 3> 2> 1> Ch3X
- because its carbocation is planar, it can approach from both faces, causing a racemic solution (front side: retention of config/ back side: inversion)
Types of rearrangement
- hydride shift
- methyl shift
Sn1 vs. Sn2
- Sn1: nucleophile strength doesn't matter
- Sn2: strong nuc required
- Sn1 substrates: 3>2 (1 and Ch3X are unlikely)
- Sn2: Ch3X> 1> 2 (3 unlikely)
- Sn1: requires a good solvent
- Sn2: may go faster in less polar solvent
- Sn1: inversion and retention
- Sn2: complete inversion
- Sn1: rearrangement
- Sn2: none
- - 3> 2>>1
- 1) unimolecular ionization to give a carbocation (rate-limiting)
- 2) Deprotonation by a weak base gives the alkene
competes with Sn1
in elim rxns, the most subbed alkene usually predominates
- - can be bulky
- - 3>2>1
- - Steps:
- -- single step; base abstracts proton on carbon neext to leaving group and the leaving group leaves, forming an alkene
- - can form multiple products based on substituted
- - Stereospecific: they must be anti to each other
E1 vs E2
- E1: base strength is unimportant
- E2: strong base
- E1: requires a good ionizign solvent
- E2: solvent polarity is not so important
E1, E2: 3> 2> 1
E1, E2: Zaitsev
- E1: no particular geometry required for slow step
- E2: coplanar arrangement (usually anti) required for the transition state
- E1: rearrangement common
- E2: no rearrangements
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