4. sikloalkana

8/18/2019 4. Sikloalkana

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John E. McMurry

www.cengage.com/chemistry/mcmurry

Chapter 4

Organic Compounds:

Cycloalkanes and theirStereochemistry

Modified by Dr. Daniela R. Radu


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! Because cyclic molecules are commonly encountered in allclasses of biomolecules:

- Proteins, Lipids, Carbohydrates, Nucleic acids

It is important to understand the nature of these molecules as

you will encounter them in the next phases of your academictraining

Why this Chapter?

! Many organic compounds contain rings of carbonatoms. Examples:

-

Prostaglandins

- Steroids


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! Cycloalkanes are saturated cyclic hydrocarbons

! Have the general formula (CnH2n)

4.1 Naming Cycloalkanes


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1) Find the parent. # of carbons in the ring. Ex. 6 C" Parent: Cyclohexane

2) Number the substituents

Naming Cycloalkanes


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! Cycloalkanes are less flexible than open-

chain alkanes

" Much less conformational freedom in

cycloalkanes

4.2 Cis-Trans Isomerism in

Cycloalkanes


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! Because of their cyclic structure, cycloalkanes have 2 faces as

viewed edge-on

“top” face “bottom” face

- Therefore, isomerism is possible in substituted cycloalkanes

- There are two different 1,2-dimethylcyclopropane isomers

Cis-Trans Isomerism in

Cycloalkanes (Continued)


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! Stereoisomerism

!

Compounds which have their atoms connected

in the same order but differ in 3-D orientation

Cis-Trans Isomerism in

Cycloalkanes (Continued)


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! Rings larger than 3 atoms are not flat

! Cyclic molecules can assume nonplanarconformations to minimize angle strain and torsionalstrain by ring-puckering

! Larger rings have many more possible conformations

than smaller rings and are more difficult to analyze

4.3 Stability of Cycloalkanes:

Ring Strain


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! Baeyer (1885): since carbon prefers to have bondangles of approximately 109°, ring sizes other than five

and six may be too strained to exist

! Rings from 3 to 30 C’s do exist but are strained due tobond bending distortions and steric interactions

Stability of Cycloalkanes: The

Baeyer Strain Theory


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! Angle strain - expansion or compression of bondangles away from most stable

! Torsional strain - eclipsing of bonds on neighboring

atoms

! Steric strain - repulsive interactions between

nonbonded atoms in close proximity

Types of Strain That Contribute to

Overall Energy of a Cycloalkane


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Cyclopropane ! 3-membered ring must have planar structure

! Symmetrical with C–C–C bond angles of 60°

! Requires that sp3 based bonds are bent (and

weakened)

! All C-H bonds are eclipsed

4.4 Conformations of

Cycloalkanes


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! In cyclopropane, the C-C bond is displaced

outward from internuclear axis

Bent Bonds of Cyclopropane


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! Cyclobutane has less angle strain than cyclopropane but moretorsional strain because of its larger number of ring hydrogens, andtheir proximity to each other

! Cyclobutane is slightly bent out of plane - one carbon atom is about25° above

! The bend increases angle strain but decreases torsional strain

Cyclobutane


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! Planar cyclopentane would have no angle strain butvery high torsional strain

! Actual conformations of cyclopentane are nonplanar,reducing torsional strain

! Four carbon atoms are in a plane

! The fifth carbon atom is above or below the plane – lookslike an envelope

Cyclopentane


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! Substituted cyclohexanes occur widely in nature

! The cyclohexane ring is free of angle strain andtorsional strain

! The conformation has alternating atoms in a common

plane and tetrahedral angles between all carbons! This is called a chair conformation


Cyclohexane


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Step 1 Draw two parallel lines, slanted downwardand slightly offset from each other. This

means that four of the cyclohexane carbons

lie in a plane.

Step 2 Place the topmost carbon atom above and to

the right of the plane of the other four, and

connect bonds.

Step 3 Place the bottommost carbon atom below

and to the left of the plane of the middle four,

and connect the bonds. Note that the bonds

to the bottommost carbon atom a parallel to

the bonds to the topmost carbon.

How to Draw Cyclohexane


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Steric Strain


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! The chair conformationhas two kinds ofpositions forsubstituents on thering: axial positions

and equatorial positions

! Chair cyclohexane hassix axial hydrogensperpendicular to the

ring (parallel to the ringaxis) and sixequatorial hydrogensnear the plane of thering

4.6 Axial and Equatorial Bonds in

Cyclohexane


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! Each carbon atom in cyclohexane has one axial andone equatorial hydrogen

! Each face of the ring has three axial and three

equatorial hydrogens in an alternating arrangement

Axial and Equatorial Positions


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Drawing the Axial and Equatorial

Hydrogens


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! Chair conformations readily interconvert,

resulting in the exchange of axial and equatorial

positions by a ring-flip

Conformational Mobility of

Cyclohexane


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! Cyclohexane ring rapidly flips between chairconformations at room temp.

! Two conformations of monosubstituted cyclohexanearen’t equally stable.

! The equatorial conformer of methyl cyclohexane is more

stable than the axial by 7.6 kJ/mol


Monosubstituted Cyclohexanes


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! Difference between axial and equatorial conformers isdue to steric strain caused by 1,3-diaxial interactions

! Hydrogen atoms of the axial methyl group on C1 are too

close to the axial hydrogens three carbons away on C3

and C5, resulting in 7.6 kJ/mol of steric strain

1,3-Diaxial Interactions


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Steric Strain in Monosubstituted

Cyclohexanes


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! Gauche butane is less stable than anti butane by 3.8 kJ/mol

because of steric interference between hydrogen atoms on the two

methyl groups

! The four-carbon fragment of axial methylcyclohexane and gauchebutane have the same steric interaction

! In general, equatorial positions give the more stable isomer

Relationship to Gauche Butane

Interactions


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! In disubstituted cyclohexanes the steric effects of both substituents must betaken into account in both conformations

! There are two isomers of 1,2-dimethylcyclohexane. cis and trans

! In the cis isomer, both methyl groups are on the same face of the ring, andthe compound can exist in two chair conformations

! Consider the sum of all interactions

! In cis-1,2, both conformations are equal in energy


Disubstituted Cylcohexanes


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! Methyl groups are on opposite faces of the ring! One trans conformation has both methyl groups equatorial and only a

gauche butane interaction between methyls (3.8 kJ/mol) and no 1,3-diaxialinteractions

! The ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactions

! Steric strain of 4 ! 3.8 kJ/mol = 15.2 kJ/mol makes the diaxial conformation11.4 kJ/mol less favorable than the diequatorial conformation

! trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in thediequatorial conformation

Trans-1,2-Dimethylcyclohexane


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Axial and Equatorial Relationships

in Disubstituted Cyclohexanes


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! Decalin consists of two cyclohexane rings joined toshare two carbon atoms (the bridgehead carbons, C1and C6) and a common bond

! Two isomeric forms of decalin: trans fused or cis fused! In cis-decalin hydrogen atoms at the bridgehead

carbons are on the same face of the rings! In trans-decalin, the bridgehead hydrogens are on

opposite faces

! Both compounds can be represented using chaircyclohexane conformations

! Flips and rotations do not interconvert cis and trans

4.9 Conformations of Polycyclic

Molecules


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Conformations of Polycyclic

Molecules (Continued)


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Draw two constitutional isomers of cis-1,2-

dibromocyclopentane?

Let’s Work a Problem


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Answer

First, we need to understand what constitutional isomer

means!the #’s of atoms, and types of atoms are the

same, just the arrangement may be different. We have

a 5 Carbon cyclic alkane, so we can only have a casewhen we have a 1,2- or a 1,3- dibromo linkage, as

these links will be symmetrical with respect to middle

carbon.


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! Many organic compounds contain rings of carbon atoms

e.g.

- Prostaglandins

-

Steroids

Organic Compounds can be

Open-Chained or Cyclic

4. sikloalkana

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