Log in to StudySoup
Get Full Access to University of Central Florida - Study Guide - Midterm
Join StudySoup for FREE
Get Full Access to University of Central Florida - Study Guide - Midterm

Already have an account? Login here
Reset your password

UNIVERSITY OF CENTRAL FLORIDA / OTHER / CHM 2210 / what's the definition of stereochemistry?

what's the definition of stereochemistry?

what's the definition of stereochemistry?


School: University of Central Florida
Department: OTHER
Course: Organic Chemistry 1
Professor: Reuben kimwomi
Term: Fall 2018
Cost: 50
Name: Orgo 1 Study Guide for Exam 2
Description: *Stereochemistry *Understanding organic reactions *Alkyl halides *Nucleophilic substitution
Uploaded: 09/28/2018
12 Pages 3 Views 4 Unlocks

greds11 (Rating: )

Ch. 5 Stereochemistry 

what's the definition of stereochemistry?

o Stereochemistry is the 3D structure of a molecule o Stereoisomers exist only when the 3D structures differ in  their arrangement, but their molecular formula remains the same (i.e. how the atoms are oriented in space

o Compare this to constitutional isomers that differ in their  connectivity of atoms

o A configuration is the specific 3D structure of a  


o When looking at mirror images, some are identical and some are  different

o Molecules are chiral if their mirror images are  

nonsuperimposable (i.e. not identical)

o Molecules are achiral if their mirror images are  

superimposable (i.e. identical)

 These usually contain a plane of symmetry, which  cuts a molecule in half, evenly, having each half as a  

reflection of the other

o Enantiomers are nonsuperimposable mirror images o Stereogenic centers are carbon atoms bonded to four different groups (tetrahedral); these may or may not be part of a ring o Molecules can have 0 or more stereogenic centers-  If there are none, the molecule is achiral

What is chiral?

 If there is one, the molecule is always chiral and  

exists as a pair of enantiomers

 If there is two or more, the molecule can be chiral or  achiral

o To designate a stereogenic center as either R or S, assign  priorities (1-4) to the groups bonded to the central carbon atom  o The larger atom gets the higher priority Don't forget about the age old question of Sociology did not become widely accepted in the USA until when?

o If two atoms are the same, move down the chain until you  spot a difference, then assign the higher priority to the  larger atom

o If no difference is recognized, this is not a stereogenic  center

o Treat a multiple bond as if it were the number of single  bonds attached to the central carbon

o Groups with priorities moving clockwise are R and groups  with priorities moving counterclockwise are S

**remember this by drawing both a capital letter R and S and watching which direction your pencil moves over the top curve of each letter** 

Enantiomers defined as what?

Don't forget about the age old question of what is the meaning of Internal attribution?

o Compounds with two or more stereogenic centers are more  complex, due to the fact that they allow for more stereoisomers  o 2n, where n equals the number of stereogenic centers

o this indicates the maximum number of stereoisomers that  are possible, but there can be fewer

o at the point of having multiple stereoisomers,  

diastereomers are recognized

 these are stereoisomers that are not mirror images of each other

o Compounds that have stereogenic centers but are also achiral  are referred to as meso compounds

o these contain a plane of symmetry

o For compounds with two or more stereogenic centers, there will  be more than one R or S configuration

o The same R and S configurations are found throughout  identical molecules

o Opposite R and S configuration are found in enantiomers o A mix of one opposite and one configuration that is the  same is found in diastereomers

o Enantiomers have identical physical and chemical properties,  except when they react with chiral molecules

o Physical properties:

 Using a polarizer, light is only allowed to pass  

through in one plane, creating plane-polarized  


 A polarimeter is then used to pass this light through a tube containing an organic substance

 Achiral compounds do not change the light at all,  making them optically inactive

 Chiral compounds rotate the light, making them  optically active

∙   The angle the light is rotated through is α and  Don't forget about the age old question of who is George Whitefield?

rotation can be clockwise (dextrorotatory

(+)) or counterclockwise (levorotatory (-))

o Two enantiomers cancel out each others rotations because  they are rotating equally, but in opposite directions

 This is called a racemic mixture/racemate

 Optically inactive

o The number of chiral molecules interacting with the  polarized light determines the observed rotation

 Further depends on the sample tube and its contents  Specific rotation [α] is used with sample tube  length (l) (in dm), temperature, wavelength, and  

concentration (c)  

∙    α/l x c

o When there is a mixture of two enantiomers, where one is  in excess, enantiomeric excess (ee)/optical purity is

used to determine how much more of one enantiomer  there is

 ee = % of one enantiomer - % of the other  Don't forget about the age old question of What are the Basics of Social Psyc?


 if two quantities are known, a different equation can  be used to find the excess

∙    ee = [α] mixture / [α] pure enantiomer x 100%

o the optical rotation and physical properties of  

diastereomers are different

 this allows for them (along with constitutional  

isomers) to be easily separated, unlike enantiomers

o Chemical properties:

 Enantiomers react at different rates when they are  reacting with chiral molecules

Ch. 6 Understanding Organic Reaction 

o Organic reactions are drawn with a single arrow

o Reagent reacts with the organic compound- may be drawn on the left side of the reaction or above the reaction arrow o The solvent and temperature of the reaction can be drawn  above or below the reaction arrow (“hv” (light) or “Δ”  (heat))

o When reactions occur without an intermediate, the steps  can be numbered above or below the arrow (indicates the  sequence and timing of the reaction- i.e. when reagents  are added, etc.)

 Usually not present on product side

o Types of reactions:

o Substitution- atom/group replaces another

 Involve σ bonds on a carbon atom (one forms, one  breaks)

o Elimination- starting material elements are “lost”

 Π bond is formed, two σ are broken

o Addition- elements are added to a starting material  Two σ are formed, π is broken

o Reaction mechanisms provided details of what and how bonds  are broken/ formed as starting material becomes product o Provide order and rate of bond breaking/formation If you want to learn more check out Multidomestic Company is what?

o Explains which products are formed

o Concerted reaction occurs in one step

 Starting material becomes product, directly

o Stepwise reaction has more than one step and involves a reactive intermediate

o Bonds are broken via homolysis (equal dividing of  electrons) or heterolysis (unequal dividing of electrons)

 In heterolysis, the atoms have different  

electronegativities, therefore the electrons will move  towards the more electronegative atom

 Both processes require energy

 Hemolysis creates uncharged intermediates,  

heterolysis creates charged intermediates

o Curved arrows are used to show the movement of the  electron pairs, a half-headed curved arrow shows the  movement of a single electron

 Radical = reactive intermediate with single lone  electron (hemolysis)

 Carbocation= two electrons go to heteroatom, none to carbon (+)-> unstable, electrophile

 Carboanion= two electrons go to carbon, none to  heteroatom (-)-> unstable, nucleophile

 Carbocations and carboanions can be intermediates  in polar reactions

o One electron from two radicals can combine to form a  bond, or an atom with a negative charge can donate two  electrons to form a bond with an atom with a positive  charge We also discuss several other topics like what is monopolistic competition?

 Energy is always released by bond formation **Table 6.1 in textbook- types of arrows in chemical  reactions**

o Dissociation energy can be used to measure bond breaking, as it is the energy needed to break a covalent bond o Enthalpy (ΔH°) is the heat absorbed or released in a  reaction

 (+) when the reaction is endothermic and energy  is absorbed; more energy is needed to break  

these bonds than what is released

 (-) when the reaction is exothermic and energy is  released; more energy is released than what is  

needed to break these bonds

 Indicates relative strength of all bonds in a  


o Bond dissociation energy is always positive

o Homolysis is always endothermic

o Stronger/ shorter bond= higher dissociation energy  As atoms increase in size, dissociation energy  decreases

o Determining enthalpy:

 Balance the equation

 Add amount of energy needed to break bonds (+)  and the amount needed to form bonds (-)

 Find the sum of all of these numbers

o A reaction equilibrium should favor the products (Keq<1) and  the reaction rate should be decent to form the products in a  reasonable time

o Consider:

 Thermodynamics- energy and equilibrium,  

comparing that of the reactants and products

 Kinetics- reaction rates (i.e. how fast the reaction occurs)

o Equilibrium constant (k) compares the amount of  starting material and the amount of product at  

equilibrium (determines equilibrium position)

 Keq>1 = equilibrium favors products, = -ΔG°

 Keq<1 = equilibrium favors reactants/starting  materials, = +ΔG°

 Determined by the relative energy of both sides of the reaction  

 Gibbs free energy (G°) -> change in free energy (ΔG°) is the difference between the energy of the  products and reactants

o Compounds are more stable when they are lower in  energy (i.e. a reaction will favor the more stable  

products when they have less energy than the  


o Even small changes can affect the overall difference  between the products and reactants at equilibrium o Reactions can be coupled, or paired, when they work  together in an unfavorable process

o The energy from one reaction powers the next reaction o Combined the two reactions form the net reaction  Depicted by adding the substances from both and  subtracting the ones that are present in both

 Add the energies from the individual steps to find  the overall energy change

o Free energy change can be affected by enthalpy changes and  entropy change (ΔS°)- measure of the change in  randomness/disorder of products and reactants

o Entropy measures the movement and randomness of a  reaction; more movement= more energy

o Increased entropy is favored

o Entropy increases when two products are made by a  single starting material

o Entropy decreases when a ring is formed by an acyclic  compound (becomes more structured and less able to  move)

 ΔG° = ΔH° -T ΔS° T= temperature in Kelvin

o Entropy is often completely omitted when it is relatively  small, the resulting equation is ΔG° ≈ΔH° 

 (-)ΔH° = product is favored (net gain in bond  


 (+)ΔH° = reactant is favored (net loss in bond  


o Energy diagrams are used to depict reactions as they occur over time

o Shows how many steps are involved, how quickly the  reaction proceeds, and compares the reactants,  

intermediates, and products

o Energy is on the y-axis, the reaction coordinate is on the x-axis

o The transition state will not ever be isolated, as it is  connected to other parts of the reaction at the “top of  the hill”

o The activation energy (EA) is the difference between  the starting materials and the transition state

 This is the minimum amount of energy in order to  break bonds, and ultimately determines how quickly  a reaction takes place

 The height of the energy barrier is also determined  by the activation energy

o Enthalpy and activation energy are independent of each other

**be familiar with identifying parts of an energy diagram and  how to analyze its data for both a one- and two-step reaction  mechanism** 

o In a two-step reaction mechanism, the rate-determining  step is the step with the highest transition state

o Kinetics studies reaction rate, or how quickly a reaction moves o The energy barrier is the amount of energy that must  be overcome for the reactants to form the products o A larger EA = a slower reaction

o The reaction will occur quicker as the concentration  increases (more collisions by more molecules)

o The reaction will occur quicker as the temperature  increases (kinetic energy increases with heat, causing  more collisions)

o Reaction rate is not impacted by reaction quantities o Free energy, enthalpy, and entropy have no control on  reaction rate

o Rate law shows the correlation between the rate and  concentration of a reaction

 Rate = k [reactants] 

o Fast reaction = large k; slow reaction = small k

o The rate equation for a one-step reaction has the  

concentration of all reactants; the rate equation for a  two-step reaction has only the concentration of reactants  from the rate-determining step

 A reaction with two reactants = bimolecular

o Rate equation order is the sum of the exponents on the  terms

o The rate-determining step indicates how quickly the  reaction occurs, this is why the concentrations of these  reactants only appear in the rate equation

o Catalysts are added to a reaction to speed it up; this remains  unchanged after the reaction and is not present in the products o They lower the activation energy needed in a reaction o Catalysts are added to a reaction in small amounts  (catalytic amount)

o Enzymes are proteins that are a type of biological  catalyst

 They contain an active site that binds to the  

reactant (substrate)= enzyme-substrate  


 Enzymes may help a reaction by holding the  

reactant for a smoother reaction; may provide sites  

with a different pH (basic/acidic) for a certain  

reaction to take place

 The enzyme releases the substrate after the  

reaction takes place and goes to find another

Ch. 7 Alkyl Halides and Nucleophilic Substitution o Alkyl halides contain a halogen (X) atom and a carbon atom  (sp3 hybridized)

o Depending on the degree of the carbon it is bonded to, the alkyl  halide can be primary, secondary, or tertiary (very important  for determining chemical reactions)

o Vinyl halides- carbon-carbon double bond attached to a  halogen

o Aryl halides-benzene ring attached to a halogen o Neither of these participate in reactions because they are  bonded to sp2 carbons

o Allylic halides- carbon- carbon double bond is adjacent to the  halogen and carbon bond

o Benzylic halides- benzene ring is adjacent to the halogen and  carbon bond

o Naming an alkyl halide:

o Identify parent carbon chain, name as an alkane o Name using other rules of nomenclature as usual o –ine changes to –o for a halogen substituent 

o –ine changes to –ide for a halogen bonded to the alkyl  group 

o Physical properties: (Table 7.1)

o Alkyl halides are weakly polar

o Have dipole-dipole interactions because of the polar bond  (C-X), even though the rest of the molecule is nonpolar o The polar C-X bone determines the overall chemistry and  reactivity of the alkyl halide

o Manly participate in substitution and elimination reactions o React with electron-rich reagents (nucleophiles) because  the carbon is electrophilic

o Nucleophilic substitution:

o Must have a halogen bonded to an sp3 carbon (alkyl group) o The halogen (X) is referred to as the leaving group o Nucleophiles (:Nu-) contain a π bond or lone pair, which  they donate to the alkyl halide

o These reactions are Lewis acid-base reactions

o A curved arrow shows the movement of electrons o Nucleophiles replace all the leaving groups

o A good leaving group is a weaker base, with a strong  conjugate acid

o It can accept electron pairs more easily when it is more  stable (X:-)

o As basicity decreases, the function of a leaving group  increases

o (examples of good leaving groups in Table 7.2 & 7.3) o Prediction of the equilibrium direction can be simply found  by comparing the basicity of the leaving group and the  nucleophile

 When the leaving group is a weaker base than the  nucleophile, the reaction favors the products

o The structure of bases and nucleophiles is very similar (i.e. they  both have a π bond or a lone pair)

o The difference is in their actions-> nucleophiles attack  carbons (i.e. electron-deficient atoms); bases attack  protons

o Parallels:

 The stronger base is the stronger nucleophile, when  two nucleophiles have the same nucleophilic atom

 A nucleophiles with a negative charge is always  

stronger than its conjugate acid

 As basicity increases, nucleophilicity increases

o Steric effects are detected when two atoms are forced  close together in a small space

 Steric hindrance is the decrease in reactivity due to bulky groups; impacts the similarities between  

nucleophiles and bases

 Steric hindrance does not affect basicity, but is  

decreases nucleophilicity  

∙ Nonnucleophilic bases are bases that are  

poor nucleophiles

o The solvent used in a substitution reaction impacts an  atoms nucleophilicity

 Polar protic solvents- are able to hydrogen bond  (along with dipole-dipole interactions)

∙ Can solvate both anions (by hydrogen bonding)

and cations (by ion-dipole interactions)

∙ As the size of an atom increases, the  

nucleophilicity increases (opposite of basicity) 

 Polar aprotic solvents- only use dipole-dipole  


∙ Only solvate cations (by ion-dipole interactions)

∙ Stronger base = stronger nucleophile 

o Understanding a nucleophilic substitution mechanism: o Identify the order of the bonds breaking and forming  Both occur at the same time (one step)

 Breaking occurs before forming (two steps)

 Forming occurs before breaking (two steps)- not  normally considered in these reactions

o Types of mechanisms:

o SN2- bimolecular, 2nd order kinetics

 Both the nucleophile and the alkyl halide appear in  the rate equation, and changing the concentration of  either one would impact the rate

 One-step: concerted reaction

∙ Bond breaking and forming occur at the same  


 Frontside attack-> nucleophile attacks from the  same side as the leaving group

∙ the configuration around the stereogenic  

center stays the same

 Backside attack-> nucleophile attacks from the  opposite side of the leaving group

∙ The configuration around the stereogenic  

center is inverted

∙ All SN2 reactions utilize backside attack  

by the nucleophile 

 Different compounds are formed as a result of both-> enantiomers

 The rate of an SN2 reaction decreases as the amount  of R groups on the carbon with the leaving group  

increases (steric hindrance)

∙ Methyl/ 1°= easiest to react (unhindered  


∙ 2°= reacts more slowly

∙ 3°= do not react

o SN1- unimolecular, 1st order kinetics

 the reaction rate is not affected by the concentration  or identity of the nucleophile

 Two-step: bond breaking occurs before bond forming  The reactive intermediates are carbocations

∙ It is trigonal planar, and sp3 hybridized with an  

empty p orbital (above and below)

 A racemic mixture is formed when one starting  material forms two products that are enantiomers of  each other

 The rate of an SN1 reactions increases as the amount  of R groups on the carbon with the leaving group  

increases (carbocation stability)

∙ 3°= easiest to react

∙ 2°= reacts more slowly

∙ Methyl/1°= do not react

o Carbocation stability:

o Can be primary, secondary, or tertiary according to the  number of R groups on the charged carbon atom

 As this number of groups increases, the stability of  the carbocation increases

o Inductive effects-electronic effects within the σ bonds  Electron-donating groups (alkyl groups) are  used to stabilize a positive charge

∙ These groups are more polarizable and more  

able to donate electrons

∙ As the R groups replace the more unreactive  

groups, the carbocation becomes more stable,  

because the positive charge is more spread out

o Hyperconjugation- overlapping an empty p orbital with  an adjacent σ bond, thus spreading the charge  

 A 3° carbocation is therefore more stable, because  the charge is more spread out (delocalized)

 Larger R groups cause a carbocation to be more  stable

o Hammond Postulate

o As a carbocation increases in stability, the rate of an SN1  reaction increases 

o The Hammond postulate estimates the transition state  energy, therefore determining the rates of two reactions   The transition state resembles the species  

(reactants/products) that is the most similar in  


∙ Endothermic= closer to products

∙ Exothermic= closer to reactants

o By lowering transition state energy-> the reaction rate  increases

->the activation energy decreases

o A reaction will occur faster when the transition state forms  a more stable product (i.e. it is lower in energy)

 Lowering product energy does not affect the  

transition state energy, because the activation  

energy is similar no matter what

o SN1 reactions:

 The rate that the carbocation is formed is affected by the stability of it

o Factors to determine when a reaction is SN1 or SN2: (Table  7.7)

o Nucleophile- strong or weak

 Strong (high concentration) favors SN2- usually have  a negative charge

 Weak favors SN1

o Leaving group- good or bad

 A good leaving group favors the rate of both reaction  types

o Solvent- protic or aprotic

 Protic favors SN1

 Aprotic favors SN2

o Alkyl halide- CH3X, RCH2X, R2CHX, R3CX (most important)  Increasing favors SN1

 Decreasing favors SN2

 Methyl/1°- only SN2

 2°- both

 3°- only SN1

Additional Study Guide Tips/Review: 


o Familiarize yourself with isomers, enantiomers, diastereomers,  and meso compounds

o Be able to locate stereogenic centers

o Properly identify S and R configurations

o What is a racemic mixture?

o What are the main types of organic reactions?

o Bond formation and bond breaking

o What is bond dissociation energy?

o Be familiar with the concepts of thermodynamics, kinetics, and  equilibrium

o What are coupled reactions?

o Be able to identify and read an energy diagram

o What are catalysts and how are they used? What are enzymes? o Understand how to name alkyl halides

o What is nucleophilic substitution?

o What is a leaving group?

o Differentiate between the two types of solvents

o Different between the two types of reaction mechanisms o What does the Hammond postulate state?

o What are the factors that determine a reactions mechanism? o Review biological nucleophilic substitution and other real world  examples of all substances throughout the three chapters

Page Expired
It looks like your free minutes have expired! Lucky for you we have all the content you need, just sign up here