Anken là gì? Cấu tạo phân tử, tính chất, ứng dụng và cách điều chế anken
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Alkenes is an important lesson of chapter 6 – Unsaturated hydrocarbons, in Grade 11 Chemistry program. In the following article, duhoc-o-canada.com will provide you with detailed general knowledge about definitions, properties, applications, its preparation as well as distinguishing alkenes from alkanes.
What is the definition of alkenes? Homology – isomerism and nomenclature
Learning about alkenes we cannot ignore their definitions. The 11th Chemistry Textbook (Vietnam Education Publishing House) has a very clear definition: “Alkenes are open-chain hydrocarbons in which there is a C=C double bond”.
Alkene homologous series
Ethylene (CH2=CH2) and the following substances with molecular formula C3H6, C4H8, C5H10… with similar properties to ethylene form a homologous series with the common molecular formula CnH2n (n >=2) called alkenes. or oletin.
Understanding alkene isomers, we will go into details of its structural isomers and geometric isomers.
Ethylene and propylene have no alkene isomers. From C4H8 onwards corresponds to a molecular formula with alkene isomers in terms of the double bond position and the carbon chain.
In an alkene molecule, the main chain is the longest carbon chain containing a C=C double bond. Alkenes in which each carbon atom in the double bond is bonded to two different groups of atoms will have a different spatial distribution of the main chain around the double bond. It is this different distribution that creates isomers about the spatial positions of groups of atoms called geometric isomers.
For example: With the structural formula CH3-CH=CH-CH3 there are geometric isomers as shown below.
What is the nomenclature of alkenes?
Alkenes nomenclature include common names and alternate names.
Called by alkanes but changed the ending “an” to “ilen”. Examples are ethylene (C2H4), propylene (C3H6), butilene (C4H8).
Alternative name for alkenes
The alternative name of the alkene is derived from the corresponding alkane name by changing the suffix “an” to “en”. From CH4 onwards, in the name of alkenes, a number must be added to indicate the position of the first carbon atom containing the double bond. The carbon chains are numbered from the side closer to the double bond.
The rules are summarized as follows:
Choose the longest chain C that contains the double bond as the main chain.
Number the main chain C from the side closest to the double bond.
Name them in the following order: Branch position number + Branch name + Main circuit C name + Double bond number + en.
Table: Alternative names and some physical constants of some alkenes
Physical properties of alkenes
Referring to the physical properties of both alkenes, we cannot ignore a few important properties as follows:
Under normal conditions, alkenes from C2H4 to C4H8 are gases; from C5H10 onwards are solid or liquid.
The melting point, boiling point, and density of alkenes increase with increasing molecular mass.
All alkenes are lighter than water (D
Alkenes are insoluble in water.
Chemical properties of alkenes
The C=C double bond consists of a bond and a bond (the bond is less stable than the bond) so it is easier to cleave, causing the characteristic chemistry of alkenes that is easy to refer to. addition reaction to form the corresponding saturated compound.
Alkene addition reaction includes hydrogen addition reaction, halogen addition reaction and HX addition reaction.
Hydrogen addition reaction of alkenes
When heated with nickel metal (or platinum/plaid) as a catalyst, alkenes combine with hydrogen to form the corresponding alkane.
CH2=CH-CH3 + H2 → CH3-CH2-CH3 (Condition: Ni, temperature)
Halogen addition reaction
Carrying out the experiment of slowly introducing ethylene into the bromine solution, the reddish-brown color of the solution is gradually fading. We have the following equation:
CH2=CH2 + Br2 (sepia) → CH2Br-CH2Br (1,2-dibromethane, colorless)
Continue pouring the bromine solution into the alkene beaker (liquid), observe that the bromine solution is discolored. We have the following reaction equation:
CnH2n + Br2 → CnH2nBr2
HX . addition reaction
Alkenes also participate in addition reactions with water, hydrogen halides (HBr, HCl, HI) and with strong acids. Some illustrative examples:
CH2=CH2 + H-OH → CH3-CH2-OH (H+ Condition)
CH2=CH2 + H-Br → CH3-CH2-Br
Alkenes with asymmetric molecular structure, when reacting with HX, can produce a mixture of 2 products. Eg:
CH3-CH=CH2 + HBr → CH3-CHBr-CH3 (2-brompropane – main product)
CH3-CH=CH2 + HBr → CH3-CH2-CH2Br (1-brompropane – by-product)
The HX addition rule, also known as Maccopnicop’s (Markovnikov) rule, follows:
The H atom will attach to the lower carbon atom, specifically C in the double bond with more H.
The halide group (X) will attach to the higher carbon atom, specifically C at the double bond with less H.
The main product of the reaction is the product that is formed according to Maccoph’s rule.
At high temperature and pressure with the right catalyst, alkene molecules can combine to form very long chains and have large molecular weights.
… + CH2=CH2+CH2=CH2+CH2=CH2+…→… -CH2-CH2-CH2-CH2-CH2-CH2-… (Conditions: Temperature, p, catalyst)
The summary equation is as follows:
nCH2=CH2 → (-CH2–CH2-)n (Polyethylene or PE)
Polymerization is a type of polymerization reaction, which is the process of joining together many similar or similar small molecules to form very large molecules, called polymers. In there:
The first substance (CH2 = CH2) is a monomer.
-CH2 – CH2- is the link of the polymer.
n is the coefficient of coincidence.
Complete oxidation and incomplete oxidation are also one of the important chemical properties of alkenes.
Complete oxidation reaction
When burned with oxygen, ethylene and its congeners will burn and give off heat.
CnH2n + 3n/2 O2 → nCO2 + nH2O
Incomplete oxidation of alkenes
To clarify the incomplete oxidation reaction of alkenes, we performed the experiment of aeration of ethylene into the KMnO4 solution. Observe that the color of the solution will fade and there is a dark brown precipitate of MnO2.
We have the following reaction:
3CH2=CH2 + 4H2O + 2KMnO4 → 3HO-CH2-CH2-OH + 2MNO4 (precipitation) + 2KOH
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How to prepare alkenes?
There is a difference between how to prepare alkenes in the laboratory and in industrial production.
How to prepare alkenes in the laboratory
In the laboratory, ethylene is prepared from the alcohol ethylene .
C2H5OH → CH2=CH2 + H2O (Condition: Condensed H2SO4, 170 degrees Celsius)
How to prepare Alkenes in industry
Industrially, alkenes are prepared from alkanes:
CnH2n+2 → CnH2n + H2 (Conditions: Temperature, p, catalyst)
Applications of alkenes
Alkenes and their derivatives are raw materials for chemical production. Some important applications of alkenes are:
Used in the manufacture of thin films, water pipes, containers… by polymerizing ethylene, propylene, butilene… to obtain polymers.
Alkenes are also used in the production of alcohols, halogen derivatives and some other substances.
Monomers are converted from ethylene to synthesize a series of polymers to meet the diverse needs of life and technology.
Distinguish between alkenes and alkanes
To distinguish alkenes from alkanes, we use bromine solutions. When bromine is poured into a beaker containing an alkene (liquid), the bromine solution becomes discolored while the alkanes do not discolor the bromine solution.
In addition to bromine, we can also use potassium permanganate (KMnO4) to identify alkenes. Alkenes discolor the solution of potassium permanganate, making it easy to recognize.
Alkenes exercises in Chemistry 11th Textbook with detailed solutions
After you have learned the theoretical knowledge about alkenes, please actively do more exercises in the textbook to review and immediately apply what you have just learned.
Exercise 2 (Chemistry Textbook 11, page 132)
How many structural isomers are there according to the formula C5H10?
A. 4; B. 5 ; C. 3 ; D. 7
The correct answer is B. 5
Exercise 3 (Chemistry Textbook 11, page 132)
Write the chemical equation for the reaction that occurs when:
a. Propylene reacts with hydrogen and heats it up (Ni catalyst).
b. But-2-en reacts with hydrogen chloride.
c. Methylpropene reacts with water with an acid catalyst.
d. but-1-en polymerization.
Exercise 4 (Chemistry Textbook 11, page 132)
Describe a chemical method for:
a. Distinguish between methane and ethylene.
b. Separation of methane from the ethylene mixture.
c. Distinguish two unlabeled flasks containing hexane and hex-1-en.
Write the chemical equation for the reaction used.
a. Methane and ethylene are passed through the bromine aqueous solution, respectively, the substance that makes the bromine aqueous solution pale is ethylene, the substance that does not make the bromine aqueous solution pale is methane.
CH2=CH2 + Br2 → CH2Br-CH2Br
Br2(dd sepia); CH2Br-CH2Br (colorless)
CH4 does not react with aqueous bromine
b. Let the gas mixture (CH4 and C2H4) pass through the excess bromine aqueous solution, C2H4 will react with the bromine aqueous solution, the remaining gas coming out of the bromine aqueous solution is CH4.
c. Similar to sentence a
In turn, hexane and hex-1-en are passed through the bromine aqueous solution, the substance that makes the bromine aqueous solution pale is hex-1-en, the substance that does not make the aqueous bromine solution pale is hexane.
CH2=CH-[CH2]3-CH3 + Br2 → CH2Br-CHBr-[CH2]3-CH3
Br2(dd sepia); CH2Br-CHBr-[CH2]3-CH3 (colorless)
Hexane does not react with aqueous solutions of bromine
Exercise 5 (Chemistry Textbook 11, page 132)
Which of the following substances discolors bromine solutions?
C. carbon dioxide
The correct answer is NO
CH2=CH-CH2-CH3 + Br2 → CH2Br-CHBr-CH2-CH3
Br2(dd sepia); CH2Br-CHBr-CH2-CH3 (colorless)
Exercise 6 (Chemistry Textbook 11, page 132)
Slowly introduce 3.36 liters of a mixture of ethylene and propylene (dktc) into the bromine solution, the color of the solution is pale and no gas is released. The volume of the solution after the reaction increased by 4.90 grams.
a. Write chemical equations and explain the phenomena in the above experiment.
b. Calculate the percent composition by volume of each gas in the original mixture.
a. Chemical equation:
CH2=CH2 + Br2 (dd, sepia) → CH2Br-CH2Br (colorless)
CH2=CH-CH3 + Br2 (dd, sepia) → CH2Br-CHBr-CH3 (colorless)
b. Let the number of moles of ethylene and propylene be x and y moles, respectively.
The volume of solution after the reaction increases is the mass of the mixture of ethylene and propylene.
N(hh) = x+ y = 3.36/22.4 = 0.15.
M(hh) = 28x = 42y = 4.9
Solving the system of equations gives us x = 0.1 mol and y = 0.05 mol.
%VC2H4 = nC2H4 = (0.1/0.15) x 100% = 66.67%.
%VC3H6 = 100% – 66.67% = 33.33%.
Hope the information shared in the above article has helped readers understand more about alkenes and apply it well in the classroom learning process. Follow duhoc-o-canada.com‘s website regularly to update articles sharing knowledge of Chemistry, Physics and Math and many other topics.
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