ORGANIC - KLEIN 3E CH.4 - ALKANES AND...

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ORGANIC - KLEIN 3E

CH.4 - ALKANES AND CYCLOALKANES

CONCEPT: ALKANE NOMENCLATURE

● Before 1919, chemists literally had to memorize thousands of random (common) chemical names.

● IUPAC naming provides a systematic method to give every chemical structure a unique, unambiguous chemical name .

CONCEPT: ALKANE PREFIXES

We will use the following set of rules to systemically name alkanes:

Rule #1. Number the longest carbon chain and assign a root name accordingly.

● If there is a tie between longest chains, choose the chain that gives _____________ substituents.

Alkane Prefixes

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Rule #2: Decide the direction of the root chain starting from the closest substituent

● If there is a tie between substituents, compare the ______________________ substituents

● If there is STILL a tie, determine direction using _____________________________.

EXAMPLE: Name the longest carbon chain and determine the direction of the root chain

EXAMPLE: Name the longest carbon chain and determine the direction of the root chain

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Rule #3: Designate numerical locations of substituents

● When one or more substituents are identical, use the prefixes ______ (2), _____ (3), ______ (4).

● Represent substituents using –yl suffix on alkane groups. (alkanes become alkyls)

EXAMPLE: Name the root chain, determine the direction of the root chain and then identify & locate all substituents

Rule #4: Name substituents in alphabetical order (prefixes don’t count toward this!)

Rule #5: Use __________ to separate numbers from numbers, _________ to separate letters from numbers.

EXAMPLE: Provide the IUPAC name for the following alkane:

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CONCEPT: COMMON SUBSTITUENTS

Although we try to use IUPAC naming as much as we can, there are a few common substituents you should know the common names for:

EXAMPLE: Name the following alkane:

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CONCEPT: CYCLOALKANES

□ Monocyclic compounds are named by attaching the prefix “cyclo-“ to the root chain.

● The root is assigned to the portion of the alkane with the greater number of carbons:

● If there is only 1 substituent, the location can be ______________.

EXAMPLE: Name the following alkanes:

□ Bicyclic compounds are composed of ______ distinct rings attached along one bond.

□ Bridged compounds are unique types of bicyclic molecules composed of ____ compound rings attached by _____

___________________________ atoms.

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CONCEPT: BICYCLICS

□ Bicyclics come in different categories: Normal and Bridged.

Nomenclature:

● A bridgehead atom must always be in the 1 position.

● Format: bicyclo[ring1.ring2.ring3]alkane

● Number from the largest ring to the smallest ring

- If it has no bridge, then the third ring just counts as _____

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CONCEPT: ALKYL HALIDES

Alkyl halides are named by naming them as a substituent before the root chain and indicating their location.

● Prefixes: -F ______________, -Cl _______________ -Br ________________ -I ______________

Alkyl halides have NO ________________ when it comes to numbering the direction of the chain.

EXAMPLE: Name the following compounds:

a.

b.

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CONCEPT: ALKENES and ALKYNES

□ Alkenes/Alkynes are named by adding the suffix modifier (-________/-________) to the end of the root.

● Alkenes/alkynes receive ___________________ in numbering alkanes

● Location is assigned to the first double bonded carbon

EXAMPLE: Name the following compound:

a.

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CONCEPT: ALCOHOL NOMENCLATURE

□ Alcohols are named by adding the modifier (-_______) the end of the root.

● Alcohols receive _______________ priority in numbering alkanes

● Locations can be donated _______________ the root “old school” or _______________ the root “new school”

EXAMPLE: Name the following compound:

a.

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CONCEPT: AMINE NOMENCLATURE

The degree of the amine directly determines how it will be named.

1o Amines: Add the suffix –amine is to the name of the alkyl substituent. If the alkyl substituent’s name ends with an –e

replace it with –amine.

EXAMPLE: Name the following 1o Amines.

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If a higher priority functional group is present then the suffix –amine changes into the prefix –amino.

EXAMPLE: Name the following multi-functional amine.

2o and 3o amines: If different alkyl groups are attached the largest alkyl group is chosen as the parent name, and the other

alkyl groups are N-substituents.

EXAMPLE: Name the following amines

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CONCEPT: DOUBLE BOND ISOMERISM

□ Cis and trans are names given to particular arrangements of double bonds or ____________

● These isomers exist because free rotation around π bonds is ______________________

● When two groups are on the “same side of the fence”, we call them ___________

● When two groups are on “different sides of the “fence” we call them ___________

EXAMPLE: How are the different substituents related to each other?

□ E and Z isomers are similar designations given to _______________________________ alkenes

EXAMPLE: Assign cis/trans isomerism to the following alkenes

The E/Z naming system allows us to assign unique names to _______ and __________substituted alkenes.

● Choose the highest priority groups on both corners of the double bond. How are they related to each other?

- If ___________, assign the letter (E)

- If ___________, assign the letter (Z)

EXAMPLE: Assign an (E) - (Z) designation to the following alkenes if applicable.

a. b.

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PRACTICE: Determine the IUPAC names of the following molecules

a.

b.

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CONCEPT: CONFORMATIONS

Most organic molecules have the ability to exist in multiple arrangements without experiencing any chemical changes.

□ Many of these arrangements exist due to the ability of ______ bonds to _____________

□ These arrangements are NOT isomers because structurally the molecule never changes.

EXAMPLE: Hexane Conformers

● These alternate arrangements are called _________________________

PRACTICE: Determine if the following pairs of molecules are isomers or conformers.

a. b.

c. d.

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CONCEPT: NEWMAN PROJECTIONS – CONFORMATIONAL ENERGY

Newman projections are drawings used to help us visualize all the conformers that can be made by rotating a ___ bond

□ The dihedral angle ________ is used to describe rotation around a single bond

● Calculated by taking the angle of the largest group on the front and back relative to each other

θ = ____ = ___________________: The two largest groups overlap each other

● _____________ energy

θ = ____ = ___________________: The two largest groups are adjacent to each other

● _____________ energy

θ = ____ = ___________________: The two largest groups are opposite to each other

● _____________ energy

EXAMPLE: Plot the following dihedral angle values with their respective energy to determine the energy diagram for the rotation of hexane along the C3 – C4 bond.

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CONCEPT: NEWMAN PROJECTIONS – METHOD

Through a series of steps, we can consistently draw accurate Newman Projections to determine conformational stability.

EXAMPLE: Draw the most energetically favorable Newman Projection for CH3CH2CH2CH2CH3 down the C2 – C3 bond.

1. Convert problem into bond line structure

2. Highlight the bond of interest

3. Draw an eyeball glaring down the length of the bond

4. Surround only the bond of interest with ALL implied hydrogens

5. Draw front carbon with 3 groups in the front and a back carbon with 3 groups in the back

6. Determine which dihedral angle would correspond

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PRACTICE: DRAWING NEWMAN PROJECTIONS

1. Draw the most energetic Newman Projection of CH3CH(C6H5)CH3

2. Draw the most stable Newman Projection of CH3CH2CH2OH through the C2 – C1 bond.

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CONCEPT: CALCULATING CONFORMATIONAL ENERGY

□ Sometimes we’ll be asked to calculate the energy barrier (kJ/mol) of rotation or of a specific interaction.

● Barrier to rotation can be calculated by memorizing other known values.

EXAMPLE: The barrier to rotation for the following molecule is 22 kJ/mol . Determine the energy cost associated with the

eclipsing interaction between a bromine and hydrogen atom.

PRACTICE: The barrier to rotation for 1,2-dibromopropane along the C1—C2 bond is 28 kJ/mol. Determine the energy cost

associated with the eclipsing dibromine interaction.

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CONCEPT: HEAT OF COMBUSTION

Heat of Combustion is a technique that blows up molecules to see how energetic they are:

___ Heat of Combustion = ____ Energy = ____ Stability

___ Heat of Combustion = ____ Energy = ____ Stability

Sources of Alkane Instability:

1. Shape: Straight chains are less stable than branched chains.

2. Strain: Found in many cycloalkanes. There are a few types of strain:

□ Angle strain exists when cyclic tetrahedral bonds are forced out of their ideal bond angle of ____________

□ Torsional strain exists when neighboring carbons possess hydrogens that overlap in space (eclipse)

EXAMPLE: Which of the following conformations of cyclohexane would have the lowest heat of combustion?

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CONCEPT: CYCLOHEXANE – CHAIRS AND POSITIONS

Although so far we have assumed rings to be planar, cyclohexane actually exists in a ______________ form to alleviate

torsional and ring strain.

Like single bonds, cyclohexane can ___________ to form two different “chair conformations” in equilibrium with each other

Chair conformations have TWO substituent positions:

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CONCEPT: CYCLOHEXANE - DETERMINATION OF CIS AND TRANS

How to draw cyclohexane:

□ Draw two slightly angled parallel lines □ Cap both ends

□ Cis or trans is based on whether the groups are facing the same __________ of the ring

● NOT based on whether they are axial or equatorial.

EXAMPLE: Name the following cyclohexane compounds:

a.

b.

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CONCEPT: CYCLOHEXANE – EQUATORIAL PREFERENCE

One of the two positions is much more crowded or ______________ ____________________ than the other.

● Rings will ALWAYS “flip” in order to accommodate the preference of their largest, bulkiest substituent. When chairs flip: □ Axials become ______________________ □ Equatorials become ______________________ EXAMPLE: This chair is not in its most stable conformation. Draw the chair flipping to accommodate equatorial preference a.

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PRACTICE: Drawing Equatorial Preference

1. Draw the MOST STABLE conformation of cis-1-tert-butyl-4-methylcyclyhexane

2. Draw the LEAST STABLE conformation of trans-1-tert-butyl-3-neopentylcyclohexane.

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CONCEPT: CYCLOHEXANE – EQUIVALENT CHAIRS

Many of the cyclohexane molecules we will draw in this chapter will be substituted.

□ In this chapter, only three things matter when drawing equivalent chairs.

● Distance between groups

● Cis vs. Trans

● Equatorial Preference (determines conformers)

PRACTICE: Determine if the following pairs of chairs are identical, conformers or different.

a.

b.

c.

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CONCEPT: CALCULATING FLIP ENERGY

Sometimes we’ll be asked to calculate the energy required (kJ/mol) to flip chairs into the axial position.

PRACTICE: Calculate the difference in Gibbs free energy in (kJ/mol) and (kcal/mol) between the alternative chair conformations of the following disubstituted cyclohexanes:

a. trans-4-iodo-1-cyclohexanol

b. cis-2-ethyl-1-phenylcyclohexane

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CONCEPT: CALCULATING CHAIR EQUILIBRIUM

We can use the difference in (∆Gº) to calculate the percentage and/or ratio of chairs at any given temperature.

● First, use ∆Gº to solve for the equilibrium constant:

*Correction: Gas Constant = 8.314

● Then, use Ke to solve for the percentage of each conformer:

PRACTICE: Estimate the equilibrium composition of the chair conformers of the following cyclohexanes at room temp:

a) cis-1,3-diethylcyclohexane

b) trans-1-methyl-3-phenylcyclohexane

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CONCEPT: DECLINS

□ Declins are specific types of bicyclic molecules. They come in two conformations of differing stability.

EXAMPLE: Draw the following declin as a chair conformation in the most stable conformation.

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ORGANIC - KLEIN 3E CH.4 - ALKANES AND...

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