Recognize the properties of alkenes relative to alkanes. Most alkynes are less dense than water (they float on top of water), but there are a few exceptions. We say that the Index of Hydrogen Deficiency is equal to the number of pairs of Hydrogens that must be taken away from the alkane to get the same molecular formula of the compound under investigation. Alkene isomers that can achieve more regular packing have higher melting and boiling points than molecules with the same molecular formula but weaker dispersion forces. Alkynes are stronger bases than water, and acetylene (ethyne) is produced in a science classroom reaction of calcium carbide with water. o What happens to enol? temperature of - They exhibit neither geometric nor optical isomerism. Cycloaddition processes involving alkynes are often catalyzed by metals. Halogen is added by anti-addition in inert solvent such as dichloromethane. Potassium hydroxide with very high temperature. Dissolving metal reduction Carbon-carbon triple bonds are highly reactive and easily broken or converted to double or single bonds. proton will add to the sp carbon bonded to hydrogen since secondary vinylic o What is the mechanism? No it is a weak base. CC licensed content, Specific attribution,,,,,'s%20rule, In addition, they do not conduct electricity. o Can cis alkene be converted into something else? occur to either sp carbon. 2. All are insoluble in water and soluble in solvents with low polarity such as other carbon to form 2 pi bonds. greater number of hydrogens. o What is the regiochemistry? Alkenes are more reactive than their related alkanes due to the relative instability of the double bond. o What happens if the same group is connected to internal alkyne? 2. Only anti addition. Ch3Br, Copyright © 2020 StudeerSnel B.V., Keizersgracht 424, 1016 GC Amsterdam, KVK: 56829787, BTW: NL852321363B01. If the alkene is asymmetric, the reaction will follow Markovnikov’s rule—the halide will be added to the carbon with more alkyl substituents. Markovnikov addition but here, product would be enol. For cycloalkenes, the carbons in the double bond are numbered as positions 1 and 2. 2 geminal dihalides products are formed since initial addition of proton can easily  Preparation of alkynes Next, the position of the double or triple bond is indicated using the position of the carbon in the bond with the lower backbone number, and the suffix for the compound is changed to “-ene” for an alkene and “-yne” for an alkyne. Calcium carbide may formally be considered a derivative of acetylene, an extremely weak acid (though not as weak as ammonia). is more electronegative than sp2 carbon of alkyl cation. All are insoluble in water and soluble in solvents with low polarity such as benzene and ether. Geminal dihalide is formed as product. However this reaction is not so much useful for the preparation of Alknes. Alkenes and alkynes are generally more reactive than alkanes due to the electron density available in their pi bonds. sodium amide followed by water. Alkenes and alkynes are useful reagents in polymer synthesis—an important industrial application. effective in stabilizing charge on vinylic cation rather than alkyl cation. o Why is the regiochemistry the way it is? 3. Bond angles are 180 (linear) oil water BOILING POINTS AND MELTING POINTS OF ALKANES For compounds containing both double and triple bonds, the “-ene” suffix precedes the “-yne,” and the compound is numbered to minimize the bond positions. The general reaction can be pictured as: [latex]\text{RC}\equiv \text{CR}'\xrightarrow{\text{KMnO}_4} \text{RCO}_2\text{H}+\text{R}'\text{CO}_2\text{H}[/latex]. Adding H2O along with the diatomic halide results in a halohydrin addition and an α-halo ketone. Markovnikov addition. Alkynes are also molecules consisting of carbon and hydrogen atoms, but instead of forming a double bond with only one sigma (σ) and one pi (π) bond, the alkyne has at least one pair of carbon atoms who have a σ and two π bonds -- a triple bond.  Synthesis Identify the Alkane molecular formula for six carbons. (less dense than water) They are isomers (tautomers) and their interconversion is called tautomerization. alkenes are less dense than water, whereas alkynes are denser than water b. alkenes are insoluble in water, whereas alkynes are soluble in water. Ethylene: A space-filling model of ethylene, the simplest alkene, showing its planar structure. o What is the difference between internal alkenes and terminal? Hydrogenation reactions typically employ a metallic catalyst consisting of platinum, nickel, palladium, or rhodium. Each carbon is sp hybridised right? Alkynes can be hydrated into either a ketone or an aldehyde form. o Can addition of HX to alkene be stopped at 1st step? Summary notes on alkyne reactions and mechanisms, INTRODUCTORY ORGANIC CHEMISTRY I (Chem 221). o Example for alkylation? Dehalogenation of tetrahalides in the presence of Zn yields Alkynes. Some are naturally occurring In regards to the syn-hydrogenation, anti is hydrogenation when one hydrogen is added from the top of the pi bond and the other is added from the bottom.  Structure of alkynes As you should know already, carbon is generally found in a tetravalent state - it forms four covalent bonds with other atoms. o What happens if excess hydrogen halide is present? Mechanism is similar to that of formation of ethane using kolbe's electrolysis. o What favors formation of terminal alkyne? o What does it do? In speculating as to what the bonding and structure could be with an index of 3 that could mean: Clearly the answer cannot be determined from the formula alone, but the formula will give important clues as to a molecule's structure.