In the compositions of Ludwig van Beethoven, C minor is commonly regarded as a special key: works for which Beethoven chose this key are felt to be powerful and emotionally stormy.
During the Classical era, the key of C minor was used infrequently and always for works of a particularly turbulent cast. Mozart, for instance, wrote only a very few works in this key, but they are among his most dramatic ones (the twenty-fourth piano concerto and fourteenth piano sonata, for instance). That Beethoven chose to write a much larger proportion of his works in this key, especially traditionally "salon" (i.e. light and diverting) genres such as sonatas and trios, was a sort of conscious rejection of older aesthetics, valuing the "sublime" and "difficult" above music that is "merely" pleasing to the ear.
Here are some quotations from commentators to this effect: the key is said to represent for Beethoven a "stormy, heroic tonality"; he uses it for "works of unusual intensity"; and it is "reserved for his most dramatic music."
Pianist and scholar Charles Rosen writes:
"Beethoven in C minor has come to symbolize his artistic character. In every case, it reveals Beethoven as Hero. C minor does not show Beethoven at his most subtle, but it does give him to us in his most extrovert form, where he seems to be most impatient of any compromise".
A characteristic 19th century view is that of the musicologist George Grove, writing in 1898:
"The key of C minor occupies a peculiar position in Beethoven's compositions. The pieces for which he has employed it are, with very few exceptions, remarkable for their beauty and importance."
Grove's view could be said to reflect the view of many participants in the Romantic age of music, who valued Beethoven's music above all for its emotional force.
Of the works said to embody the Beethovenian "C minor mood", probably the canonical example is the Fifth Symphony. Also notable is that for most of Beethoven's C minor works, the most frequently occurring key for the slow movement is A flat major, the submediant, which provides a calm contrast.
Here is a list of works by Beethoven in C minor that are felt to be characteristic of how Beethoven used this key:
Cantata on the Death of Emperor Joseph II, WoO 87 (1791)
Piano Trio, Op. 1, No. 3 (1793)
Piano Sonata, Op. 10, No. 1 (1795-8)
Piano Sonata, Op. 13, "Pathétique" (1798)
String Trio, Op. 9, No. 3 (1798)
Piano Concerto No. 3 (1800)
String Quartet, Op. 18, No. 4 (1800)
Violin Sonata, Op. 30, No. 2 (1802)
Symphony No. 3, second movement, "Funeral March" (1803)
32 Variations in C minor, WoO 80 (1806)
Coriolan Overture, Op. 62 (1807)
Fifth Symphony (1808)
Choral Fantasy, Op. 80 (1808)
String Quartet No. 10, Op. 74, scherzo movement (1809)
Piano Sonata No. 32, Op. 111 (the last piano sonata, 1822)
Other minor keys
The works by Beethoven in C minor hardly exhaust the set of emotionally stormy minor-key works by this composer; some useful comparisons would include the piano sonatas Op. 2, No. 1 and Op. 57 (both in F minor), the String Quartet, Op. 95 (also in F minor), the final movements of Beethoven's only two pieces in C-sharp minor (the "Moonlight" Sonata and the String Quartet, Op. 131), and two of his most famous D-minor works: the "Tempest" Sonata (that its slow movement is in the submediant is also characteristic of Beethoven's C-minor mood) and the Ninth Symphony.