Cyclobutane, numbering atoms

Cyclobutane has less angle strain than cyclopropane but has more torsional strain because of its larger number of ring hydrogens. As a result, the total strain for the two compounds is nearly the same—110 kj/mol (26.4 kcal/mol) for cyclobutane versus 115 kj/mol (27.5 kcal/mol) for cyclopropane. Experiments show that cvclobutane is not quite flat but is slightly bent so that one carbon atom lies about 25° above the plane of the other three (Figure 4.5). The effect of... 

Even condensed structures are awkward for cyclic molecules, and a streamlined way of drawing structures is often used in which cycloalkanes are represented by polygons. A triangle represents cyclopropane, a square represents cyclobutane, and so on. Carbon and hydrogen atoms aren t shown explicitly in these structures. A carbon atom is simply understood to be at every junction of lines, and the proper number of hydrogen atoms needed to give each carbon four bonds is supplied mentally. Methylcyclohexane, for instance, looks like this ... 

Cyclopropane (Fig.J) is a flat molecule in respect of C-atoms, with the hydrogen atoms situated above and below the plane of the ring, so it has no conformational isomers. Cyclobutane can form three distinct shapes-a planar shape and two butterfly shapes (fig.K). Cyclopentane can also form a number of shapes or conformations. The planar structures for cyclobutane and cyclopentane are too strained to exist in practice because of eclipsed C-H bonds. 

A cycloaddition reaction most commonly involves two molecules reacting to form two new sigma bonds between the end atoms of their pi systems, resulting in the formation of a ring. The product has two more sigma bonds and two fewer pi bonds than the reactants. The reactions are classified according to the number of pi electrons in each of the reactants. Thus, the reaction of two alkenes to form a cyclobutane derivative is termed a [2 + 2] cycloaddition reaction, and the reaction of a diene with an alkene to form a cyclohexene derivative is termed a [4 + 2] cycloaddition reaction ... 

To compare the properties of alkanes, alkenes, and alkynes, you will be working with compounds that have the same number of carbon atoms. First, you will construct and compare butane, trans-2-butene, 2-butyne, and cyclobutane. You will use a graph to compare the boiling points of each compound. Next, you will use what you have just observed to predict the relative boiling points of pentane, trans-2-pentene, 2-pentyne, and cyclopentane. You will construct and compare these structures and graph their boiling points. 

Diels-Alder reactions are classified as [4 + 2] cycloadditions, and the reaction giving the cyclobutane would be a [2 + 2] cycloaddition. This classification is based on the number of electrons involved. Diels-Alder reactions are not the only [4 + 2] cycloadditions. Conjugated ions like allyl cations, allyl anions and pentadienyl cations are all capable of cycloadditions. Thus, an allyl cation can be a 2-electron component in a [4 + 2] cycloaddition, as in the reaction of the methallyl cation 6.2 derived from its iodide 6.1, with cyclo-pentadiene giving a seven-membered ring cation 6.3. The diene is the 4-electron component. The product eventually isolated is the alkene 6.4, as the result of the loss of the neighbouring proton, the usual fate of a tertiary cation. This cycloaddition is also called a [4 + 3] cycloaddition if you were to count the atoms, but this is a structural feature not an electronic feature. In this chapter it is the number of electrons that counts. 

Cyclopropane and cyclobutane can be opened by aggressive agents. The same agents also attack the hydrocarbon chain [13], so that three- and four-membered hydrocarbon rings are not suitable monomers. Cyclic compounds with a higher number of ring atoms are even less suitable because of the lower ring strain. 

The isodesmic reaction conserves the number of bonds of a given formal type. This is accomphshed by choosing reference molecules that contain two heavy atoms, preserving the formal bond between them. The four C-C bonds of cyclobutane are preserved in the reference as four ethane molecules. Cyclobutene, having one C=C and three C-C bonds, needs a molecule of ethene and three molecules of ethane as reference. The two C-0 bond and two C-C bonds of oxetane translate into two molecules of ethanol and two molecules of ethane. Balancing the reaction to conserve all other bonds leads to the isodesmic Reactions 3.9i-3.11i. 

The simplest member of the class is the [l]-ladderane, a small and familiar strained molecule known as cyclobutane. The strain in a [n]-ladderane increases with the number of fused rings and the introduction of multiple bonds." The [2]-ladderane derivative, Dewar benzene, is highly unstable and converts readily to the conjugation-stabilized counterpart, benzene. Depending on the stereochemistry of bridgehead atoms in the fiised-ring system, [njladderanes... 

Cation XV opens a number of routes because the quaternary nitrogen atom elicits a high degree of electrophilicity in all its vicinal carbons. Of the three a carbons in XV, the methylene carbon of the cyclobutane portion of the molecule should be the more favorable site for bimolecular nucleophilic attack by the methanol solvent, owing to the release of ring strain. This attack would directly yield the observed products III and XII. Supporting evidence for the existence of XV is found in the isolation of XVIII [Eq. (3) of Scheme 40.3] during the photolysis of I and butadiene in methanol. ... 

Compounds 7 and 8 (cyclobutanes are usually drawn in this style) are termed cis and trans, respectively. Further, 7 and 8 are diastereoisomers, though neither is chiral this is significant in that diastereoisomers encountered previously, e.g. (R, R) and meso tartaric acids, have at least one chiral member. Indeed, this observation is general for disubstituted saturated cyclic hydrocarbons provided that the number of carbon atoms in the ring, n, is even and that the substituents are located at carbon atoms 1 and 1 + nil). For cyclobutanes this corresponds to C(l) and C(3), and for cyclohexanes C(l) and C(4). The stereochemistry of 1,2-disubstituted cyclobutanes is analogous to that of cyclopropanes. 

A considerable number of naphtho[l,8-Z c]cyclobutanes was synthesized where the bridging atom between the 1,8-positions of the naphthalene structure was carbon... 

Cyclobutanes are characterized by lower retention volumes than five- and six-membered cycloolefins with the same number of carbon atoms. 

Cyclic compounds that have a ring of carbon atoms, which are not aromatic, are named using the prefix cyclo. For example, cyclobutane is a cyclic alkane with four carbon atoms. The atoms in the ring are numbered so that the smallest numbers indicate the position of substituents. 

---