2120 ME 2 – Chemistry – Flashcards

-ethane has relatively ree rotation around the carbon-carbon bond

-such rotation leads to the formation of different conformers

-ethane has two conformers

Relative Stability of Cycloalkanes

-heats of combustion per CH2 unit reveal cyclohexane (658.7KJ/mol) has no ring strain and other cycloalkanes have come ring strain

-cyclopentane and cycloheptane are second most stable

-cyclopentadecane also very very low

(659 KJ/mol)

angle strain is caused by bond angles different from 109.5 degrees

-tortional strain is caused by exlipsing C-H bonds on adjacent carbons

-cyclopropane has both a high angle and tortional strain

****molecules can bend to relieve tortional strain***

Chair Conformation

-has no ring strain

-all bond angles are 109.5 and all C-H bonds are perfectly staggered

Boat Conformation

-is less stable because of flagpole interactions and tortional strain

Twist Conformation

-intermediate in stability b/w the boat and chair

-there are axial and equatorial hydrogen atoms

-axial hydrogens are perpendicular to the average plane of the ring

-equatorial hydrogens lie around the perimeter of the ring

**C-C bonds and equatorial C-H bonds are all drawn in sets of parallel lines

-the axial hydrogens are drawn straiht up and down

Methyl cyclohexane is more stable with the methyl equatorial

-an axial methyl has an unfavorable 1,3 diaxial interaction with axial C-H bonds 2 carbons away

-A 1,3-diaxial interaction is the equivalent of 2 gauche butane interactions

GENERALLY

-substituted cyclohexane is more stable with the substituent equatorial

-these can exist as pais of cis-trans stereoisomers (diastereomers)

-CIS- groups on same side of ring

-TRANS- groups on opposite side of ring

-Trans-1,4-kimethylcyclohexane prefers a trans-diequatorial conformation***

 -Cis-1,4-dimethylcyclohexane exists in an axial-equatorial conformation

-if one group is larger than the other, the large one will exist in the equatorial position (when 1,3)

1. acid (-COOH)

 2. ester

3. amide (-CONR2)

 4. nitrile (-CN)

5. Aldehyde (-CHO)

6. Ketone (-CO-)

7. Alcohol (-ROH)

8. Phenol (ArOH)

9. Thiol (SH)

10. Amine (-NR2)

11. ether (ROR)

12. alkene

13. alkyne

14. alkyl, aryl, halide, etc

All

Elephants

Are

Nice

And

Keen

Although

Pheasants

Thwart

All

Elephants

 All

All

All

-take the alkane name, drop the -e and add -ol

-when it is priority, the carbon bonded to "OH" is #1 on the ring

-take alkane name and add thiol

-ie. ethanethiol

-ie. 2,3-pentanedithiol

SMELL

-ethanethiols- the smell added to natural gas or propane

-butanethiol- skunk scent .. detectable at 10ppb... causes nausea

-removal of thiol smell can be done by adding H2O2 or CL2 bleach

SULFIDES

-contains RSR groups respectively

-never cited as principal groups and common nomenclature is often used

-take alkane name of longest chain as principal group and treat remainig SR as alkylthio group

-ie. 3-methylthiononane

-contains ROR group

-never cited as principal groups and commen nomenclature is often used

-take alkane ame of longest chain as principal group and treat remaining OR as alkoxy group (ie. replace -ane with -oxy)

ie. ethoxy ethane

ie. 2-ethoxy-2-methylpropane

-take alkane name, drop -e and add -amine

- if more than one group on amine, principal name comes from group containing the longest chain, the other is treated as a substituent

Aldehydes

-many common names are used for simple aldehydes (ie. acetone, acetaldehyde)

-take alkane name, drop the -e and add -al

ie. octanal

Ketones

-take alkane name, drop the -e and add -one

BOTH

-when attached to a ring, suffix = carboxaldehyde

-

-when a carbonyl group is a substituent, its position is designated by oxo

****aldehyde takes priority over ketone

-common nomenclature is widely used for simple acids... e. acetic acid

-take alkane name, drop the -e and add -oic acid

-butanoic acid = rancid butter

-for acids attached to rings, add carboxylic acid to the alkane name

NOTE: priority rating to determine principal group if more than one heteroatom in molecule

acids>aldehyde>ketone>OH>SH

take acid name, drop the -ic and add -ate

-nae the substituent on the carboxylate oxygen first, and name it as an alkyl or aryl group

2-methyl-2-butenyl ethanoate is the flabor in juicy fruit gum.... flavor of raspberris, bananas, and pineapple also (or smell?)

-when attached to rings, keep alkane name and add carboxylate

ie. methyl 2-iodocyclopentanecarboxylate

lactones

-cyclic esters... cmmon nomenclature used

-ie. gamma or beta lactone.. (see slide)

-Acid Halides

-take acid name, drop -oic and add -oyl plus halide

-ie. pentanoyl chloride

-when attached to rings, named as alkanecarbonyl halide

-ie. cyclopentanecarbonyl chloride

ANHYDRIDES

(hydrogens lost)

-take acid name and add anhydride

-if R1=R2, just name one side

-if R1does not =R2, cite two parent acids in alphabetical order

-ie. butanoic propanoic anhydride

NITRILES
-take alkane name and add nitrile

-ie. 3-methylbutanenitrile

-when attached to rings, keep alkane name and add carbonitrile

-ie. 3-methylcyclohexanecarbonitrile

AMIDES

-take name, drop the -oic and add -amide

-ie. 3-chlorobutanamide

-substitution on nitrogen (other than H) is designated with the letter N

-ie. N,N-dimethylbutanamide

-when attached to rings, keep alkane name and add carboxamide

ie. 2-ethylcyclobutanecarboxamide

LACTAMS
-cyclic amides

-common nomenclature used (beta/gamma.. see slide)

-nomenclature for benzene derivatives follows the same rules used for other substituted hydrocarbons

-some monosubstituted benzenes have well established common names (phenol, anisole, toluene)

-for disubstituted benzenes, the positions of the substituents may be designated in two ways:

a)IUPAC system: numerical designations are made in the same manner as other ompound classes;

b)older, popular system

o (ortho)[1,2-]

m (meta) [1,3-]

p(para) [1,4-]

**can't used o/m/p if more than 2 groups

;

-when there is a principal group on benzene, the principal name is used;

-sometimes benzene is a substituent

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COMMON DERIVATIES OF PHENOL

3-methylphenol = meta-cresol

1,3-benzenediol = resorcinol (antiseptic)

1,4-benzenediol = hydroquinone

urushiol = poison ivy.. causes itching

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O-phenylphenol = lysol

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chiral molecule - not superposable on its mirror image

-can exist as a pair of enantiomers

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pair of enantiomers - a chiral molecule and its mirror image

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achiral molecule - superposable on its mirror image

-does not have enantiomer pairs

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**a molecule /w a single tetrahedral carbon bonded to four different groups will always be chiral

-a molecule /w more than one tetrahedral carbon bonded to four different groups is not always chiral

-****switching two groups at the tetrahedral center leads to the enantiomeric molecule in a molecule with one tetrahedral carbon

-keep any two the same and switch other two

;

Stereogenic center

(modern name for chiral)

-an atom bearing groups of such nature that an interchange of any two groups will produce a stereoisomer

-carbons at a tetrahedral stereogenic center are designated with an asterisk (*)

;

;The biological Importance of Chirality

-the binding specificity of a chiral receptor site for a chiral molecule is usually only favorable in one way

PLANES OF SYMMETRY

-an imaginary plane that bisects a molecule in such a way that the two halves of the molecule are mirror images of each other

-a molecule with a plane of symmetry cannot be chiral

Use R/S system

-also called the cahn-ingold-prelog system

-the four groups attached to the stereogenic carbon are assigned priorities from highest to lowest

-atoms directly attached to the stereogenic center are compared

-atoms with higher atomic number are given higher priority

;

-the molecule is rotated to put the lowest priority groups back

-if the groups descend in priority in clockwise, then R

-if descent in priority counterclockwise, then S

;

special case:

-if the lowest priority group points forward (towards you) no need to rotate, just reverse answer

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*groups with double or triple bonds are assigned priorities as if their atoms were duplicated or triplicated

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**if both carbons have same stereo name, then molecules are identical (in a neo)

-if one is R and one S, then the two molecules are enantiomers

-enantiomers have almost all identical physical properties (melting point, boiling point, density)

-however, they rotate the plane of plane-polarized light in equal but opposite directions

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plane polarized light

-oscillation of th electric fireld of ordinary light occurs in all possible planes perpendicular; to the; direction of propagation

- if the light is passed through a polarizer only one plane emerges

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-an empty sample tube or one containing an achiral molecule will not rotate the plane-polarized light

-an optically active substance (ie. one pure enantiomer) will rotate the plane polarized light

-the amount the analyzer needs to be turned to permit liht through is called the observed rotation alpha

-the standard value specific rotation ([alpha]) can be calculated

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******

-i the analyzer is rotated to the right the rotation is positive and the molecule is dextrorotary (right handed-ness)

-if the analyzer is rotated to teh let, the rotation is negative and the molecule is levorotatory (left handedness)

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specific rotation = observed rotation/ concentration times length of tube

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[alpha] = alpha/(c)(l)

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-when you have a 1:1 mixture of enantiomers

-the net optical rotation will be zero

-designated as (+/-)

2ⁿ=#isomers

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-look at example in notes

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-the maximum number of stereoisomers available will not exceed 2ⁿ where n is equal to the number of stereogenic centers

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-the max number of diastereomers a chiral molecule can have is 2ⁿ-2

-sometimes molecules with 2 or more stereogenic centers will have less than the maximum number of stereoisomes

-this is due tothe existence of meso compounds

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meso compound:

-achiral despite the presence of stereogenic centers, its is

-not optically active

-superposable on its mirror image

-has a plane of symmetry

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NAMING

-the molecule is manipulated to allow assignment of each stereogenic center separately

;

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FISCHER PROJECTION FORMULAS

-can be used to give a 2D representation of chiral molecules

-vertical lines represent bonds that project behind the plane of the paper

-horizontal lines represent bonds that project out of the plane of the paper

*old fashioned kiss

-may have planes of symmetry

-if the case, then not chiral and not optically active

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Effectss of Hyridization on Acidity

-hydrogens connected to orbitals with more s character will be more acidic

-s orbitals are smaller and closer to the nucleus that p orbitals

-anions in hybrid orbitals with more s character will be held more closely to the nucleus and be more stabilized

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-willing to trade pi bond for sigma bond... triple bonds are therefore unstable

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triple bond = stronger acid and weaker base

single = stronger base and weaker acid

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REACTIONS

4 types:

susbstitutions

additions

eliminations

rearrangements

homolysis

-bond breaks evenly

;

heterolysis

-one atom takes both electrons

;

heterolytic reactions almost always occur at polar bonds

-the reaction is often assisted by formation of a new bond to another molecule

*bond to long pair or long pair to bond

;

-reaction can occur to give a carbocation of carbanion depending on the nature of Z

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carbocations

-carbocations have only 6 valence electrons and a positive charge

- they readily react with any Lewis base to satisfy the otet around the electron-deficient carbon

;

carbanions

-carbanions have 8 valence electrons and a negative charge

-they readily react with any Lewis acid to satisfy the octet around the electron rich carbon

electrophiles

-(electron-loving reagents)

-seek electrons to obtain a stable valence shell

-are electron deficient themselves (ie carbocations)

;

nucleophiles

-seek a proton or some other positively charged center

-are electron rich themselves (ie carbanions)

;

-any organic compound containing an atom with a lone pair (O,N) can act as a base, ie a nucleophile

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-pi electrons are loosely held and available for reaction with strong acids (double to single bonds)

The Substitution Reaction of tert-Butyl Alcohol

-the reaction has 3 steps

-all steops are acid-base reactions

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Step 1

-is a bronsted acid-base reaction

Step 2

-involves heterolytic cleavage of a bond

-is a lewis acid base reactionin reverse

Step 3

-is a lewis acid base reaction with chloride acting as a lewis base and the carbocation acting as lewis acid

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seee notes!!!