Physical properties of alkenes

Alkenes resemble alkanes in most of their physical properties. The lower molecular weight alkenes through C4H8 are gases at room temperature and atmospheric pressure. 

The dipole moments of most alkenes are quite small. 7 mong the C4HS isomers, 1-butene, cA-2-butene, and 2-methylpropene have dipole moments in the 0.3-0.5 D range fran.y-2-butene has no dipole moment. Nevertheless, we can learn some things about alkenes by looking at the effect of substituents on dipole moments. 

Experimental measurements of dipole moments give size, but not direction. We normally deduce the overall direction by examining the directions of individual bond 

Higher atomic number takes precedence over lower. Bromine (atomic number 35) outranks chlorine (atomic number 17). Methyl (C, atomic number 6) outranks hydrogen (atomic number 1). 

When two atoms directly attached to the double bond are identical, compare the atoms attached with these two on the basis of their atomic numbers. Precedence is determined at the first point of difference  

Experimental measurements of dipole moments give size, but not direction. We normally deduce the overall direction by examining the individual bond dipoles. With alkenes the basic question concerns the alkyl groups attached to C=C. Does an alkyl group donate electrons to or withdraw electrons from a double bond This question can be approached by comparing the effect of an alkyl group, methyl for example, with other substituents. 

Arrange the following in order of increasing dipole moment. 

Alkyl groups donate electrons to sp -hybridized carbons of an alkene. 

Comparison of -r ui 1 Melting Points Table 11-1 of Irenes and Alkanes  

Polarization in Alkenes Alkyl Groups are Inductive Electron Donors 

Another consequence of the electron-attracting character of the sp carbon is the increased acidity of the alkenyl hydrogen. Whereas ethane has an approximate p Ta of 50, 

Higher ranked groups are on the same side of the double bond the configuration is Z. 

A table on the inside back cover lists some of the more frequently encountered atoms and groups in order of increasing precedence. You should not attempt to memorize this table, but should be able to derive the relative placement of one group versus another. When naming compounds according to the E, Z system, the descriptor is placed in parenthesis and precedes the rest of the name. 

Number the chain as shown and list the substituents alphabetically. The compound is (Z)-l-bromo-l-chloropropene. 

The physical properties of alkenes are similar to those of alkanes. Alkenes which contain 2-4 carbon atoms are gaseous, 5-17 carbon atoms are liquids, and higher alkenes are solids at room temperature. 

Boiling points increase as the number of carbon atoms increases. Branching is an important factor which decreases the boiling point of alkenes. The densities of alkenes are a little bigger than those of the same sized alkanes. 

The chemical properties of alkenes are very different from those of alkanes because of the double bond (— C = C —) in the structure. Double bond contains a sigma bond and a pi bond. Since electrons in pi bonds are bonded less strongly than in sigma bonds. This makes alkenes chemically reactive combustion, substitution, oxidation and polymerization reactions are all undergone by alkenes. 

Alkenes produce CO2 and H2O when they are burnt with a sufficient amount of oxygen. The general combustion reaction of alkenes is  

6 L oxygen is required to burn 0.25 mole of an alkene at STR What is the molar mass of this alkene (C 12 H l) 

Most physical properties of alkenes are similar to those of the corresponding alkanes. For example, the boiling points of 1-butene, cw-2-butene, tra/t5-2-butene, and n-butane are all close to 0°C. Also like the alkanes, alkenes have densities around 0.6 or 0.7 g/cm. The boiling points and densities of some representative alkenes are listed in Table 7-2. The table shows that boiling points of alkenes increase smoothly with molecular weight. As with alkanes, increased branching leads to greater volatility and 

1 TABLE 7-2 Physical Properties of Some Representative Alkenes J 

Name Structure Carbons Boiling Point (°C) Density (g/cm ) 

Like alkanes, alkenes are relatively nonpolar. They are insoluble in water but soluble in nonpolar solvents such as hexane, gasoline, halogenated solvents, and ethers. Alkenes tend to be slightly more polar than alkanes, however, for two reasons The more weakly held electrons in the pi bond are more polarizable (contributing to instantaneous dipole moments), and the vinylic bonds tend to be slightly polar (contributing to a permanent dipole moment). 

In a cis-disubstituted alkene, the vector sum of the two dipole moments is directed perpendicular to the double bond. In a trans-disubstituted alkene, the two dipole 

The physical properties of the homologous series of alkenes are similar to those of the homologous series of alkanes Alkenes have densities ranging from 0.6 to 0.8 g cm (Table 

Alkenes are either nonpolar or very slightly polar. Thus, they are insoluble in water but soluble in nonpolar solvents such as hexane. They are also soluble in diethyl ether and halogenated solvents. 

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The Orbital Description of the Alkene Double Bond 286 7-3 Elements of Unsaturation 287 7-4 Nomenclature of Alkenes 289 7-5 Nomenclature of Cis-Trans Isomers 291 Summary Rules for Naming Alkenes 293 7-6 Commercial Importance of Alkenes 294 7-7 Stability of Alkenes 296 7-8 Physical Properties of Alkenes 302... 

Most physical properties of alkenes are similar to those of the corresponding alkanes. For example, the boiling points of but-l-ene, m-but-2-ene, fran.v-but-2-ene, and n-butane... 

The physical properties of alkenes, alk)mes, and aromatic compoimds are very similar to those of alkanes. They are nonpolar. As a result of the Tike dissolves like" rule, they are not soluble in water but are very soluble in nonpolar solvents such as other hydrocarbons. They also have relatively low boiling points and melting points. 

The physical properties of alkenes are very similar to those of the alkanes. They are nonpolar, insoluble in water, less dense than water, and soluble in nonpolar solvents. Alkenes are quite reactive, and their characteristic reaction is addition to the double bond. Three important addition reactions are bromination (an example of halogenation) to give a dibrominated alkane, hydration to produce an alcohol, and the reaction with H—X to give an alkyl halide. The addition of HjO and H—X are governed by Markovnikov s rule. 

In what ways are the physical properties of alkenes similar to those of alkanes ... 

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