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Alkene - properties of unsaturated hydrocarbons

Plastic straws
Source: domena publiczna.

Link to the lesson

Before you start you should know
  • how to write the general formulas of the homologous series of hydrocarbons;

  • how to write the molecular formula of a hydrocarbon with the specified number of carbon atoms;

  • which compounds are called alkanes, and which ones are called alkenes and alkynes.

You will learn
  • what physical properties of alkenes are, on the example of ethene;

  • what chemical properties of alkenes are, on the example of ethene.

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Nagranie dźwiękowe abstraktu
Task 1
Wyjaśnij kluczowe pojęcia związane z tematem abstraktu.
Wyjaśnij kluczowe pojęcia związane z tematem abstraktu.

Construe all possible models of a butene molecule and compare them with one another.

Alkene

Chemical formula

Boiling point
(°C)

Melting point
(°C)

Density (g·cmIndeks górny -3)
(20°C and 1013 hPa)

ethene

CIndeks dolny 2HIndeks dolny 4

-103.7

-169.0

0.5678

propene

CIndeks dolny 3HIndeks dolny 6

-47.6

-185.2

0.505

butene
(but‑1‑ene)

CIndeks dolny 4HIndeks dolny 8

-6.2

-185.3

0.588

butene
(trans‑but‑2‑ene)

CIndeks dolny 4HIndeks dolny 8

0.8

-105.5

0.599

butene
(cis‑but‑2‑ene)

CIndeks dolny 4HIndeks dolny 8

3.7

-138.9

0.616

Alkenes containing a double bond at a carbon atom other than the first in the chain may have the form of cis- isomers (both methyl groups – the substituents are on the same side of the plane in which the binding lies) and the trans‑isomers (the substituents are at opposite sides of the plane).
The properties of these compounds depend on the type of bonds formed by carbon atoms in molecules of organic compounds (Sawicka Jolanta, Janich‑Kilian Anna, Cejner‑Mania Wiesława, Chemical tables, Podkowa, Gdańsk 2004.).

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Two bottles labeled: ethene and propene
Source: Tomorrow Sp. z o.o., licencja: CC BY 3.0.

Chemical properties of alkenes

Task 2

Formulate a research question and hypothesis before conducting “Testing the combustibility of alkenes”. After the experiment, do the interactive exercises in the summary to consolidate what you have learned.

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Analysis of the experiment: „Testing the combustibility of alkenes”. Research question (Uzupełnij). Hypothesis (Uzupełnij).
Badanie palności alkenów – pokaz nauczycielski
Experiment 1
Research problem

How does ethene burn?

Hypothesis

Select one of the presented hypotheses, and then verify it.

Ethene burns explosively.
Ethene burns completely.
Ethene burns incompletely.

You will need
  • a test tube with collected ethene, closed with a stopper,

  • torch.

Instruction
  1. Remove the stopper from the tube filled with ethene.

  2. Keep the burning torch near the outlet of the tube.

  3. Observe the occurring changes.

Summary

With restricted oxygen access incomplete combustion takes place and the flame is smoky:

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Exercise 1
Fill in the reaction equation. Dostępne opcje do wyboru: water, dwa o dwa, oxygen, dwa ha dwa o. Pierwsze równanie: ce dwa ha cztery dodać tu uzupełnij strzałka w prawo dwa ce o dodać tu uzupełnij. Drugie równanie: ethene dodać tu uzupełnij strzałka w prawo carbon monoxide dodać tu uzupełnij.

Ethene is a flammable gas. With unrestricted air access, it burns with an even flame. In this case complete combustion occurs:

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Exercise 2
Dostępne opcje do wyboru: oxygen, dwa ha dwa o, water, trzy o dwa. Fill in the reaction equation. Pierwsze równanie: ce dwa ha cztery dodać tu uzupełnij strzałka w prawo dwa ce o dwa dodać tu uzupełnij. Drugie równanie: ethene dodać tu uzupełnij strzałka w prawo carbon dioxide dodać tu uzupełnij.
Task 3

Formulate a research question and hypothesis before conducting the experiment “Testing the reactivity of ethene”. Write your observations and conclusions. Pay attention to whether and how ethene reacts with bromine.

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Analysis of the experiment: “Testing the reactivity of ethene”. Research question (Uzupełnij). Hypothesis (Uzupełnij).
Badanie reaktywności etenu
Experiment 2
Research problem

Does ethene react with bromine?

Hypothesis

Select one of the presented hypotheses, and then test it.

Ethene does not react with bromine.
Ethene reacts with bromine.

You will need
  • two test tubes (including one with ethene),

  • stopper,

  • bromine solution.

Instruction
  1. Fill 1/3 of the first test tube with bromine solution (control).

  2. Fill the second test tube (with collected ethene) with the same amount of bromine solution. Close the test tube with a stopper and mix well by shaking.

  3. Observe the occurring changes.

  4. Compare the color in both tubes. 

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Summary
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Observations (Uzupełnij). Conclusions (Uzupełnij).

The reaction of ethene with bromine in anhydrous conditions proceeds according to the equation:

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CIndeks dolny 2HIndeks dolny 4 + BrIndeks dolny 2 → CIndeks dolny 2HIndeks dolny 4BrIndeks dolny 2

As a result of this reaction, the multiple bond breaks and a saturated compound dibromoethane forms (the di - prefix informs that there are two bromine atoms in the molecule). This type of reaction is called the addition reaction.

Under appropriate conditions, hydrogen may be added to alkenes. This reaction is called the hydrogenation reaction.

The reaction of ethylene with hydrogen proceeds according to the equation:

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CIndeks dolny 2HIndeks dolny 4 + HIndeks dolny 2 → CIndeks dolny 2HIndeks dolny 6

In 1869, the Russian chemist Vladimir Markownikov proposed a rule defining the direction of the reaction of addition of elements to the double bond between carbon atoms.This principle is that, generally, in addition reactions to the -C=C- bond, occurring in many organic compounds (e.g., alkenes), lower electronegativity atoms or groups attach to it from two carbon atoms to which it was previously attached more atoms or groups with electropositive properties.

The rule is not always satisfied for all addition reactions to the -C=C- bond. Strictly speaking, it is fulfilled only when this reaction occurs through the ionic mechanism, monomolecular. When this reaction proceeds according to another mechanism, it often joins contrary to the Markownikov's rule.

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The Markownikov's Rule
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Exercise 3
Indicate true statements. Możliwe odpowiedzi: 1. Saturated hydrocarbons can be distinguished from unsaturated hydrocarbons by using potassium permanganate., 2. In the addition reaction of ethene with hydrogen chloride we can obtain a chemical compound called dichloroethane, 3. The addition reaction of hydrogen with unsaturated hydrocarbons is called a hydrogenation reaction., 4. The physical properties of alkenes do not depend on the length of the carbon chain., 5. Ethene burns with a pale blue flame.
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Exercise 4
Indicate true statements. Możliwe odpowiedzi: 1. As the number of carbon atoms in the alkene molecule increases, their melting point increases., 2. As the number of carbon atoms in the alkene molecule increases, their boiling point decreases., 3. As the number of carbon atoms in the alkyne molecule increases, their melting point increases., 4. As the number of carbon atoms in the unsaturated hydrocarbon molecule increases, their boiling point increases.
Task 4

Design an experience in which you use bromine water to distinguish between saturated and unsaturated hydrocarbons. Remember to write down the research problem and the hypothesis. Then try to do a list of all chemical reagents and laboratory equipment, that are necessary to complete the experiment. Create instructions and write down observations and conclusions.

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Research problem (Uzupełnij). Hypothesis (Uzupełnij). Chemical reagents and laboratory equipment (Uzupełnij). Instructions (Uzupełnij). Observations (Uzupełnij). Conclusions (Uzupełnij).

Summary

  • The physical properties of alkenes and alkynes are similar to the physical properties of the alkanes. They change as the number of carbon atoms in the chain increases.

  • Alkenes and alkynes are chemically reactive, flammable, and undergo addition reactions.

  • Alkynes are more reactive than alkenes.

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Grafika przedstawiająca tablicę szkolną, na której znajdują się zdania do dokończenia. Finish selected sentences. 1 It was easy for me (uzupełnij). 2 It was difficult for me (uzupełnij). 3 Today I learned (uzupełnij). 4 I understood that (uzupełnij). 5 It surprised me (uzupełnij). 6. Today I found out (uzupełnij. 7. I was interested in (uzupełnij). 8. I still have to repeat (uzupełnij.
Source: GroMar Sp. z o.o., licencja: CC BY-SA 3.0.
Homework
Task 5.1

Explain the differences between ethane, ethene and ethyne molecules.

Keywords

alkenes, unsaturated hydrocarbons, physical properties of alkenes, chemical properties of alkenes

Glossary

addition reaction
addition reaction
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Nagranie dźwiękowe słówka

reakcja addycji – reakcja przyłączenia atomów innych pierwiastków, np. fluorowców (chloru, bromu, jodu), do cząsteczki z wiązaniem wielokrotnym, bez wydzielenia produktów ubocznych

hydrogenation reaction
hydrogenation reaction
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Nagranie dźwiękowe słówka

reakcja uwodornienia – reakcja przyłączenia cząsteczki wodoru do cząsteczki z wiązaniem wielokrotnym, bez wydzielenia produktów ubocznych