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3 -- Part 3: STRUCTURE AND STEREOCHEMISTRY OF ALKANES
The lowest-energy alkane has all the internal carbon-carbon bonds.
The chain has a zigzag shape.
The internal carbon-carbon bonds undergo rotation at room temperature.
The zigzag structure is made of Gauche conformations.
We draw alkane chains in a zigzag structure to represent the most stable arrangement.
The formation of long-chain alkyl groups is important in the chemistry of lipids, with implications for their melting points and for nutrition.
Draw a representation of the most stable part of 3-methylhexane.
The antibiotics we use to treat diseases and the carbohydrates we eat are all cyclic.
The general formula loss of consciousness is caused by the fact that each one has exactly cycloalkanes and twice as many hydrogen atoms as carbon atoms.
Because a ring has no ends and no hydrogens are needed to cap into the blood, cyclopropane goes for an acyclic alkane.
The chemistry of cyclopropane is no longer cal properties.
The compounds are nonpolar and are used as an anesthetic because of their boiling points and melting points.
When mixed with air, the physical properties of ether and can cycloalkanes cause explosions because they are held in a more compact shape.
Some common cycloalkanes have their physical properties listed in Table 3-4.
Similar to acyclic alkanes, cycloalkanes are named.
Substituted cycloalkanes have alkyl groups named as substituents.
No numbering is needed if there is only one substituent.
If there are two or more substituents on the ring, the ring carbons are numbered to give the lowest possible numbers.
The numbering begins with one of the substituted ring carbons and continues in the direction that gives the lowest possible numbers to the other substituents.
The substituents are listed in alphabetical order.
When the numbering can begin with either of two substituted ring carbons, begin with the one with more substituents and the one that is first alphabetically.
When the cyclocyclic portion of the molecule contains more carbon atoms than the cyclic portion, it is sometimes called aalkyl substituent.
The IUPAC names for the compounds should be given.
Give the formula for each of the compounds by drawing the structure.
1,1-diethyl-4-(3,3-dimethylbutyl)cyclohexane Open-chain (acyclic) alkanes are free to assume any of the hundreds of single bonds they have.
Alkenes have double bonds that prevent rotation and give rise tocis and trans isomers with different orientations of the groups on the double bond.
There are similarities between cycloalkanes and alkenes.
A cycloalkane has two faces.
The cis-trans isomer of but-2-ene and 1,2- dimethylcyclopentane are compared.
Simple rotation about the bonds is not enough to convert the dimethylcyclopentane.
The thicker lines indicate the parts of the molecule that are closer to the viewer.
The cis and trans isomers can be drawn.
The following cycloalkanes should be given IUPAC names.
The most common rings contain between five and six carbon atoms.
The properties of many important organic compounds are determined by the stabilities and conformations of these rings.
In 1905, Adolf von Baeyer was awarded a Nobel Prize for his work on the relative stabilities of cyclic molecules.
The bond angles of the carbon atoms in acyclic alkanes were determined by Baeyer.
The rise to torsional strain can be attributed to this eclipsing of bonds.
The size of the ring is the most important factor in determining the amount of ring strain.
Before we talk about the ring strain of cycloalkanes, we need to consider how it is measured.
In theory, we should measure the total amount of energy in the compound and subtract the amount of energy in a strain-free reference from the bonds.
The amount of extra energy should be the same.
Extra energy can be released in the combustion if the compound has ring strain.
The molar heat of cyclohexane is nearly twice that of cyclopropane.
The heat of combustion is divided by the number of CH2 groups to compare the relative stabilities of cycloalkanes.
The result is the amount of energy per group.
We can compare the relative amounts of ring strain in the cycloalkanes.
There are some simple cycloalkanes listed in Table 3.
The refer ence value of 658.6 kJ (157.4 kcal) per mole of CH2 groups comes from an unstrained long-chain alkane.
The values show a lot of ring strain.
The amounts of ring strain for cyclopentane, cycloheptane, and cyclooctane are smaller than for cyclohexane.
The pattern of ring strain will be explained in detail.
There are two factors that contribute to the large ring strain.
The angle strain needed to compress the bond angles from 10 to 60 degrees is the first thing you need to know.
When the bond angles are different, the 3 orbitals are weakened.
3 orbitals can't point at each other, and they overlap at an angle to form weaker bonds.
The ring is a three membered one.
Each unit is per mole.
The large total ring strain helps to account for the torsional strain in cyclopropane, which is not as great as its angle strain.
Cyclopropane is more reactive than other alkanes.
The cyclopropane ring release 115 kJ (27.6 kcal) per mole of ring strain provides an additional driving force for these reactions.
The trans isomer is larger.
Draws can be used to explain the difference in stability.
The carbon atoms were hybridized.
The total ring strain is caused by all the carbon-carbon bonds being eclipsed.
The cyclobutane is slightly folded.
The Newman projection shows that folding gives partial relief from bonds.
The ring strain in cyclobutane is similar to that in cyclopropane, but is distributed over four carbon atoms.
If cyclobutane were square, it would have 90 degree bond angles.
The bonds need to be nixed as in cyclopropane.
To reduce this strain, cyclobutane assumes a slightly folded form.
The relief of some of the torsional strain appears to compensate for a small increase in angle strain for these smaller bond angles.
Draws can be used to explain the observations.
If cyclopentane had a shape similar to a regular pentagon, its bond angles would be close to the same angle.
A structure with a cyclopentane-like ring would need all the bonds to be eclipsed.
Cyclopentane is assumed to be slightly different from the real thing.
The properties and strain are dependent on these puckered "envelope" conformations.
The shape is not fixed but undulates by the thermal reactions of the two genes.
As the molecule undulates, the envelope seems to move around the ring.
The ring systems of cyclohexane are more common than other cycloalkanes.
Carbohydrates, steroids, plant products, pesticides, and many other important compounds have cyclohexanelike rings that are critically important to their reactivity.
The abundance of cyclohexane rings in nature is probably due to their stability and the fact that they are predictable.
Nature probably forms more six-membered rings than all other ring sizes combined.
There must be no angle strain and no eclipsing of bonds in cyclohexane.
The bond angles of a regular hexagonal would be 120 degrees rather than 109.5 degrees.
The bonds on adjacent CH2 groups would be overshadowed by the torsional strain of a planar ring.
The cyclohexane ring cannot be a straight line.
If you assume a puckered conformation, cyclohexane can achieve bond angles and staggered conformations.
The angles between the carbon-carbon bonds are all in the chair.
The Newman projection shows all the bonds in staggered configurations.
The "footrest" methylene group is folded upward in the boat.
There is a strain on the boat.
The hydrogens on the ends of the boat are interfered with by this.
If you twist your model slightly, the hydrogens move away from each other and the bonds are reduced.
Even though the twist boat is less energy efficient than the symmetrical boat, it is still more energy efficient than the chair.
The twist boat is often intended when someone refers to the boat.
Most of the cyclohexane sample are in chairs.
The chair to the boat takes place by the footrest of the chair flipping upward and is critical for their activities.
The highest-energy point in this process is the one where steroids fit into their footrest.
It converts between the boat and chair forms when the correct fit is used.
There are two different kinds of carbon-hydrogen bonds if we could freeze cyclohexane in a chair.
The bonds are directed parallel to the axis of the ring.
The bonds point out from the ring.
Each carbon atom has two hydrogen atoms, one directed upward and one downward.
C2 has an upward and downward bond.
The pattern changes.
The odd-numbered carbon atoms have bonds that are either up or down.
The even-numbered carbons have bonds that are up and down.
The pattern of alternating bonds is helpful for predicting the conformations of substituted cyclohexanes.
There are certain rules that should be followed to show the actual positions and angles of the substituents on the ring.
Make a cyclohexane ring with your models, put it in a chair and use it to follow along with the discussion.
The angles of the bonds in the model should correspond to the angles in the drawing.
The carbon-carbon bond framework can be drawn with two parallel lines slightly slanted and offset.
The atoms at the end of these bonds are in a plane, and they are the "armrests" of our chair.
Draw the lines connecting the carbons to the armrests.
The two lines that connect the headrest carbon should be parallel to the two lines that connect the footrest.
You can notice the pairs of carbon-carbon bonds with three distinct slopes by comparing this drawing with your model.
The chair can be drawn with the headrest to the left or the right.
Draw it both ways.
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